When President Trump fired FBI Director James Comey in May, he pledged to move quickly to find a replacement. He kept that promise, taking less than a month to announce that he had chosen former federal prosecutor Christopher Wray to fill the job. But it took Trump nearly another three weeks — until last Monday — to formally send Wray’s name to the Senate for approval.

Wray’s nomination isn’t the only one moving slowly. Five months into his presidency, Trump lags far behind his predecessors in filling senior positions in his administration, a delay that some experts say is holding back Trump’s agenda. As of June 28, nominees for 46 out of 561 key jobs in the Trump administration had been confirmed by the Senate, according to the nonpartisan Partnership for Public Service. That’s far fewer positions than had been filled by the previous four presidents at this point in their administrations. At the same time in his first term, President Obama had appointed (and the Senate had confirmed) 183 people.

Trump has repeatedly blamed Senate Democrats for holding up his appointments. It’s true that the Senate has moved somewhat more slowly to confirm Trump’s nominees than it did to confirm Obama’s: 43 days on average for Trump versus 35 days for Obama. But the bigger problem is that Trump has put forward far fewer nominees than past presidents had at this point in their terms. The Partnership for Public Service and The Washington Post are tracking key appointments that require Senate confirmation; as of Friday, 384 of the 564 positions had no formal nominee and 130 had been nominated but not confirmed. Four appointments had been announced by Trump but not yet submitted to the Senate.

“The biggest obstacle [to filling the positions] is that the White House hasn’t submitted nominations,” said Darrell West, director of governance studies at the Brookings Institution, a Washington think tank.

Experts point to several possible reasons for the delays, including reported disagreements within the administration over who should fill some top positions. The wealthy business executives appointed to some top jobs have required lengthy vetting processes, and some nominations have been killed outright. The Washington Post last month, citing former federal officials and other sources, reported that Comey’s firing and the ongoing investigation into possible Russian interference in last year’s election was also making hiring more difficult.

The White House didn’t respond to calls for comment, but Trump has said previously that “in many cases, we don’t want to fill those jobs.”

Experts on both the left and right, however, said the large number of vacancies could make the administration less effective. Politico this month reported that many Cabinet secretaries are spending large chunks of time meeting with Trump, in part because they don’t yet have deputies. Dozens of ambassador positions have yet to be filled, leaving embassies in countries such as Afghanistan and France with acting ambassadors. Key roles at the Treasury Department remain vacant, including officials in charge of White House priorities such as tax reform and financial deregulation. At the Education Department, the roles overseeing civil rights, policy development and technical education are empty. And despite Trump’s focus on blue-collar workers, he has yet to fill positions in the Labor Department dealing with employee training, veterans employment and mine safety.

“His policies aren’t going to go anywhere if staff is not implemented,” said Tom Fitton, president of the conservative watchdog group Judicial Watch. Fitton said that the holdup in nominations is a “concern shared widely.”

Moving slowly to fill positions could also be hurting Trump in another way: It puts more power in the hands of the career government employees, whom Trump has reportedly accused of trying to undermine his agenda.

“It really empowers the career bureaucracy,” West said. “If you don’t have the senior political leadership within a particular agency, it’s going to be the long-term staffers who make decisions, and that’s challenging for the administration because most people don’t support the Trump agenda.”