(CNN) Last October, two House Republican committee chairmen announced their plans to get to the bottom of the FBI's decision-making in 2016 -- namely its handling of the Hillary Clinton email investigation.

"Decisions made by the Department of Justice in 2016 have led to a host of outstanding questions that must be answered," Reps. Bob Goodlatte, chairman of the House Judiciary Committee, and Trey Gowdy, chairman of the House Oversight Committee, said at the time.

But eight months later, the joint investigation has yet to answer many of those questions -- and prompted a growing amount of grumbling from Republicans themselves.

Several Republicans who serve on the two panels told CNN they are dissatisfied with the pace of the investigation, saying the chairmen have been slow to schedule witness interviews and have held marathon sessions with just four witnesses who have shed little light on both the Clinton investigation and the start of the FBI's Russia probe in 2016. They gripe that the chairmen have not pushed the Justice Department harder on their subpoenas to get access to more documents. And some want to have their witness hearings in public settings, rather than behind closed doors as they have been so far.

"The major frustration is with a lack of tenacious spirit in getting documents and scheduling interviews with appropriate witnesses," Rep. Mark Meadows of North Carolina, a member of the House Oversight Committee and chairman of the conservative House Freedom Caucus, told CNN on Monday. "While the time frame may be viewed as appropriate, conservative members and the American people feel like much more could be done."

"No," Rep. Matt Gaetz, a Florida Republican, said when asked if he was satisfied by the investigation. "Because we haven't been tough about getting the documents. We keep playing this game, and it's starting to look like it's a low-energy investigation if we don't start holding people in contempt and beginning impeachment proceedings in the absence of compliance with congressional oversight."

The comments underscore the headwinds Republicans have faced in trying to unearth new dirt about the Clinton email investigation, which has already been thoroughly reviewed by the Justice Department's inspector general. GOP lawmakers complain they are looking at a smaller amount of records and interviewing fewer witnesses than the inspector general did, and some say the committees have not spent enough energy trying to uncover what they view as malfeasance in the FBI's Russia probe in 2016.

Amid the criticism, the House Judiciary Committee is stepping up its pace. On Tuesday, the panel is slated to vote on a resolution by conservative lawmakers directing the Justice Department to provide documents about surveillance on Trump associates in the 2016 elections, although the committee had to mark up the resolution or it would go straight to the House floor for a vote.

On Wednesday, the FBI agent at the center of controversy in both inquiries -- Peter Strzok -- will meet with the House Judiciary Committee behind closed doors for a deposition after being slapped with a subpoena by Goodlatte, even though Strzok had voluntarily agreed to be interviewed.

And on Thursday, the panel will hold a public hearing with FBI Director Christopher Wray and Deputy Attorney General Rod Rosenstein about the inspector general's report, giving the Republicans an opportunity to dress down the FBI and Justice Department leaders about their document requests.

The Republican chairmen say they understand the frustration from their GOP colleagues — and hoped things would have moved faster to this point — but they point the finger at the Justice Department. And Goodlatte told CNN on Monday they have succeeded in forcing the Justice Department to create a process for turning over hundreds of thousands of documents.

"I'm not satisfied," the Virginia Republican told CNN. "But we have made a lot of progress with the establishment of this office at the Department of Justice where we have access to close to a million documents now. We can see them all unredacted, we can identify the ones we want produced. ... I won't say it's complete compliance, but we are getting a lot of information we need."

Several Republicans on the committees have privately said that Strzok should be grilled in public, so his testimony can't be spun by either side. Goodlatte added that Strzok will face a public hearing at a later date

"They'll get that opportunity, but we're going to interview him first," he said. "Sometime soon we'll have a public hearing — that's the intention."

But President Donald Trump joined his conservative allies to protest the closed-door hearing on Twitter Monday evening.

"The hearing of Peter Strzok and the other hating frauds at the FBI & DOJ should be shown to the public on live television, not a closed door hearing that nobody will see," Trump tweeted. "We should expose these people for what they are - there should be total transparency!"

Lawmakers say they're at a loss over why more witnesses haven't come through -- including former FBI lawyer Lisa Page, who traded thousands of text messages with Strzok in 2016, including some decidedly anti-Trump messages. Trump and his conservative allies have seized on their texts as a key reason to make the case that the Clinton and Russia probes have been tainted.

"We've wanted to interview and depose Lisa Page months ago," Rep. Jim Jordan of Ohio, a member of both committees, said when asked if he was satisfied with the investigation. "We are finally getting there," he added, referring to Strzok's Wednesday interview. "But we should have been interviewing people a lot sooner than this -- I have been very clear about that."

Asked why it's taken so long, Jordan said: "I don't do the subpoenas, I don't do the interviews -- that has to go through the chairmen of the committee. ... We just keep asking, and we just keep waiting."

Meadows added: "The fact that we are just now interviewing Peter Strzok, and we have yet to schedule Lisa Page ... would suggest a less-than-aggressive time frame to complete the investigation."

So far, the committees have also held only four interviews as part of the FBI probe. After high-profile interviews in December with then-FBI Deputy Director Andrew McCabe and January with James Comey's former chief of staff James Rybicki, the committees went dark on interviews for several months.

They've kicked back up with a new series of interviews of FBI officials in the past month, including FBI director of counterintelligence Bill Priestap and John Giacalone, who was the FBI's top national security official during the first part of the Clinton email investigation. Sources in both parties acknowledge that the witness interviews have provided little information beyond what is already publicly known.

"I personally don't think they've turned up a whole lot in these interviews, and obviously, they've been trying to hold these interviews in part to discredit the FBI and Jim Comey," said Rep. Raja Krishnamoorthi, an Illinois Democrat on the House Oversight Committee who has attended all the witness interviews. "The interviews are just not producing evidence or testimony that will support their hypothesis."

More interviews are likely on tap, as the committees earlier this month sent the Justice Department a list of nearly a dozen FBI and Justice Department officials they want to interview, which included Strzok, according to a letter obtained by CNN.

Yet there are ample concerns that many of these key witnesses have already been interviewed by the inspector general, with Meadows saying that "rehashing something" already investigated by the inspector general is of "little value." Instead, the investigation, several conservative argue, should look at the actions taken over the Russia probe in 2016.

Leaders of the committee say the witness interviews have been slow because they've been trying to get documents first, arguing the Justice Department has refused to fully comply with their demands.

"The drama does not help anyone," Gowdy, a South Carolina Republican who has yet to issue a subpoena as chairman of the Oversight Committee this Congress, said last week. "So if you're going to produce it, produce it -- if you're not going to produce it let us know, that's helpful information, too. And then (House Speaker Paul Ryan) has to decide a number of things. But when you push the issue, you're not getting the documents, you're going to court."

The fight with the Justice Department over documents -- which has also involved House Intelligence Chairman Devin Nunes, a California Republican, battling over subpoenas for his investigation into the start of the FBI's Russia investigation in 2016 -- has prompted calls for the firing of Rosenstein from several of Mueller's most vocal critics in the House.

By making the threats, they've managed to extract a number of key documents from the Justice Department and FBI, but they've also let numerous deadlines come and go without full compliance with their subpoenas.

Gaetz, like several of his fellow conservatives, argues it's time to target Rosenstein, even by possibly impeaching him, and he wants his colleagues to take a firmer approach.

"If we were really serious about conducting oversight, we would be demanding the documents," Gaetz said.