You aced the interview, your résumé sings, but in the end, you didn't get the job. Chances are, you'll never know why.

It is a painful conundrum of the job search process: Rejected candidates want to understand why they didn't get hired, but employers, fearing discrimination complaints, keep silent. And those who do speak up offer little more than platitudes.

Without specifics, candidates are left to repeat the same mistakes, while hiring managers complain they're swamped with applicants who miss the mark.

Peter and Maria Hoey

"You don't know how to adjust going forward," says technology professional Lisa Roberson. When she wasn't selected for a job in her field a few years ago, she emailed one of the people who had interviewed her to find out why.

The response: Someone "more suited" to the job had been hired. "Well, I could have guessed that," said Ms. Roberson, who works in health-care IT.

Such exchanges frustrate job seekers, especially those who have been searching for long periods and desperately want some insight into how they are viewed by hiring managers.

Providge Consulting, a Delaware-based consulting firm, has a policy to keep candidates apprised at every step of its hiring process and scores candidates on a range of criteria to keep its decisions as objective as possible.

But when the reasons for a rejection can't be boiled down to more clear-cut measures like experience or education, HR managers "attempt to minimize those conversations," said Tara Teaford, director of operations. That may mean offering a vague response, adding that the company will reach out if appropriate positions arise in the future.

"Most of it is trying to protect ourselves from potential litigation," says Ms. Teaford. "Once you cross the line between objective and subjective, it gets very, very challenging."

And many of the firms that want to provide feedback have their hands tied by company lawyers.

Employers were put on notice in late 2012 when the Equal Employment Opportunity Commission identified discrimination in hiring practices as one of its priorities for the next three years, partly out of a recognition that few job seekers have the resources to hire a lawyer and press their claims through civil courts, according to Amy Fratkin, an employment lawyer. That means individual complaints will be more likely to result in lawsuits brought by the EEOC if the agency establishes a pattern of discrimination by the employer.

Linda Jackson, a partner with employment law firm Littler Mendelson, says she advises her clients against offering specific feedback to job candidates. For instance, telling someone he has too much experience for a particular job might be interpreted as age discrimination, she said. "Is it the basis for a claim? It might or it might not be," she says.

Then there is the discomfort of relaying hard-to-hear information. Some hiring managers are so uncomfortable at the prospect of these conversations that they refuse to bring their business cards to interviews, says Amelia Merrill of Risk Management Solutions Inc., a risk-modeling firm in Silicon Valley.

Despite how awkward it can be, Ms. Merrill expects her recruiters to call finalists to let them know they weren't hired, giving those applicants a chance to ask for more information. She wants even rejected candidates to leave thinking they want to work there.

On rare occasions, she added, a rejected candidate will argue with the recruiter or insist he was the right pick for the job.

Of course, lots of candidates don't seek feedback: HR managers put the number of those who request it at around 10%. But of those who do, barely any get it. Only 4.4% of more than 2,000 job candidates surveyed in 2012 by the Talent Board, an organization dedicated to improving companies' recruiting practices, said they received specific feedback from hiring managers and recruiters.

But it raises the question, will the gap ever be bridged?

"If you want an efficient labor market, you have to have people understand where their talents are best used," says Elli Sharef, co-founder of HireArt, a website that matches job seekers and employers through video interviews and assessment tests.

After hearing from hundreds of frustrated job seekers, Ms. Sharef recently decided to try offering feedback, despite some trepidation from her lawyer.

In May, HireArt emailed 127 job seekers who had submitted video interviews for jobs in educational technology and offered the chance for a 15-minute personalized critique from Ms. Sharef herself. The 21 available slots were filled in less than 10 minutes.

Most appreciated the assistance, but overall, reactions to the feedback varied. One person complained that 15 minutes wasn't enough time for the session. Others admitted they hadn't given much thought to what they could contribute to the prospective employer, which was the most common criticism.

HireArt has since decided to offer a limited number of weekly feedback sessions. But as the company weighs scaling up the service to more users, it is also wrestling with questions about how people absorb and use constructive criticism. It can be difficult to hear "negative information about yourself, especially when you're already in a vulnerable position," says Ms. Sharef.

Companies' job-application software could provide another source of feedback, albeit automated, suggests John Sullivan, a management professor at San Francisco State University.

These applicant-tracking systems, which are used by almost every large employer, score candidates based on rough measures like the number of keyword matches between a job description and a résumé. Employers could theoretically send candidates their scores, says Mr. Sullivan.

"If you scored 90 out of 100, you might apply again later. But if you scored a 20, you know you applied for the wrong job," he said. So far, none of the companies for which he has recommended this, have adopted it. They fear it will generate additional questions from applicants or reveal too much about the keyword-matching process, he says.