Image copyright Getty Images

Despite all the Trump-related good fortune that has been showered on Hillary Clinton over the past week, there has been a bit of a dark lining on her silver cloud. She still can't seem to come up with an effective response to questions surrounding her use of a private email server while secretary of state.

On Sunday Mrs Clinton gave her first television interview since accepting the Democratic nomination last week. Perhaps as part of her strategy to reach out to disaffected Republicans, she chose to appear in that bastion of conservatism, Fox News, for the first time in five years.

Not surprisingly, it didn't take long for the email topic to come up.

When host Chris Wallace asked the Democrat if she was being truthful to the American people when she said that she did not send classified emails on her personal account, Mrs Clinton cited FBI Director James Comey's public statement on the investigation into the matter as corroboration.

"Director Comey said my answers were truthful, and what I've said is consistent with what I have told the American people, that there were decisions discussed and made to classify retroactively certain of the emails," she said.

Image copyright Getty Images Image caption FBI Director James Comey has declined to comment on whether Hillary Clinton lied to the American people about classified material on her email server

Comey's "truthful" description only applied to Mrs Clinton's conversations with the FBI, however - not her public statements. According to the FBI's findings, Mrs Clinton did indeed send emails that contained information that were classified at the time of transmission - a few of which contained "top secret" information.

Mrs Clinton also appeared to direct some of the blame for the whole imbroglio to her subordinates. After acknowledging that she "made a mistake" not using separate email addresses for personal and work messages, she said she had relied on the "judgements of the professionals" with whom she worked.

"In retrospect, maybe some people are saying, well, among those 300 people, they made the wrong call," she said. "At the time, there was no reason in my view to doubt the professionalism and the determination by the people who work every single day on behalf of our country."

Media fact-checkers have offered harsh reviews of Mrs Clinton's response. The Washington Post gave it four "Pinocchios", its lowest rating. Politifact labelled her statement "pants on fire".

"Clinton is cherry-picking statements by Comey to preserve her narrative about the unusual setup of a private email server," writes the Post's Glenn Kessler. "This allows her to skate past the more disturbing findings of the FBI investigation."

More on the Clinton campaign

Why they hate Hillary in Trump country

Are the Democrats too confident?

Three reasons Hillary could lose

The challenge for Mrs Clinton is that to answer Wallace's question accurately would require her to acknowledge one of two unpleasant truths.

Either she knew there was classified information on her email server and wilfully tried to deceive the American people, or she wasn't "particularly sophisticated with respect to classified information", in the words of Comey, revealing a lack of knowledge that plays directly against the claims of hyper-competency that undergird her presidential campaign.

There's little doubt the email controversy has had a negative effect on Mrs Clinton's standing with the US public - particularly in surveys that ask whether she is "honest and trustworthy".

Following Comey's press conference in which he announced that Mrs Clinton would not face criminal charges but was "extremely careless" in her handling of classified material, the then-presumptive nominee's approval ratings dropped and head-to-head presidential matchups with Donald Trump narrowed to a statistical tie.

In a mid-July poll conducted by ABC and the Washington Post, 57% of respondents said the email controversy made them "very" or "somewhat" worried about how Mrs Clinton would handle presidential responsibilities if she were elected.

A CNN poll found Mrs Clinton's favourability ratings dropped from 48% favourable on 1 May to only 39% on 24 July.

Her numbers have rebounded somewhat, thanks in part to a successful Democratic National Convention and Mr Trump's smorgasbord of controversial statements and questionable campaign moves.

The matter is far from resolved, however, given that the State Department has reopened its investigation into whether Mrs Clinton or her aides violated government policy and should receive some form of sanction. Before too long, the Democratic nominee may once again be called onto offer an explanation for her actions under the glare of the national media spotlight.

Image copyright Getty Images Image caption Donald Trump's ongoing controversies are burying any fallout from Hillary Clinton's Fox News interview on Sunday

Until Mrs Clinton finds a way to answer questions about her email server without making statements that blame-shift, appear nakedly evasive or are easily debunked by fact-checkers, her campaign will continue to have a glaring soft spot.

Maybe Mr Trump will be able to refocus and exploit the weakness. Maybe he won't. But even if Mrs Clinton prevails in November, she will need a base level of support and trust from the American people to govern effectively.

At least at this point, there's no guarantee that she'll have it.