Being out of work for two years or longer makes candidates harder to place than people with a criminal record, recruiters say. Gen Y, take note: A history of job hopping is a deal-breaker, too.

FORTUNE -- "One of the most frustrating elements of a job search is the silence," notes Art Papas, CEO of Boston-based recruiting software maker Bullhorn. "It can be maddening to apply for a position and then hear nothing about whether you're even being considered for an interview -- and, if not, why not."

In an effort to shed some light on what prospective employers are thinking when they read your resume, researchers at the company asked about 1,500 recruiters and hiring managers to reveal what makes them turn thumbs down. The top five answers:

1. Job hopping. About 40% of those surveyed said a history of frequent job changes, including leaving any previous employer within a year of getting hired, is likely to disqualify an applicant. That's a big hurdle for Millennials, who are notorious for changing jobs, on average, every two years.

2. Outdated skills. Having "skills that are no longer in demand" puts employers off, according to about one in three (31%) of those surveyed, while 28% cited being "out of touch with modern workplace technology."

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3. Getting fired. Most recruiters and hiring managers ranked having been sacked as "severely damaging" to candidates' chances of landing a new job, which isn't too surprising. But what came next on the list is more disconcerting: A chronic illness or disability, followed closely by having taken an extended maternity or paternity leave, which the study says "was deemed more harmful to career prospects than leaving the workforce to get an advanced degree or even being laid off."

4. Age. About 70% of poll respondents say candidates in their 30s are in bigger demand than any other age group. Interestingly, though, longer experience does count for something: Headhunters and hiring managers see "greater demand for candidates in their 40s than for those in their 20s."

5. Unemployment. A candidate who's been out of work for six months to a year is hard to place, according to 36% of those surveyed, and 28% said the same of someone with any gaps in his or her employment history.

But the real trouble starts at two years. "Recruiters admit it's easier for them to place someone with a (non-felony) criminal record in a new job than to place someone who has been unemployed for two years," the researchers note. That's discouraging news, when you consider that the number of long-term unemployed Americans is now larger than at any time since the Great Depression.

Nonetheless, Art Papas insists that it's possible to overcome the stigma of a long spell of joblessness. "We've spoken with recruiters who referred us to people who did find new jobs after two years or more of unemployment, and we've asked them how they did it," he says. "What we discovered was that it's not so much a matter of how long you've been unemployed, as a question of how long you've been out of the game."

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What's the difference? "It helps to keep developing your skills and staying active in your field while you're job hunting," Papas explains. One woman he spoke with "took online courses, earned new certifications, did a couple of unpaid internships, and attended trade group conferences and other networking events, so she had something recent to talk about in job interviews," he says. "If you stay current and involved, and keep your resume up-to-date to reflect that, it's shows employers you're motivated and a self-starter."

Papas adds that people who are out of work are often embarrassed about it, so they don't reach out to friends, relatives, and former colleagues to find out about possible job leads. "There's nothing shameful about unemployment, when 23 million other people are in the same boat," he says. "And networking and referrals are how you'll get that next job. People want to help. Often it's making that first call [to someone in your network] that's the hardest. Once you get some momentum going, it gets easier."