The job search process is time consuming and stressful for most job seekers. You spend hours customizing your cover letter and resume to a job description. Then you send it off… and wait. There is nothing more frustrating than applying for a position that you feel you’re well qualified for and never hearing back on why you weren’t being considered.

To better understand what prospective employers are looking for when reviewing resumes and the red flags that make them push certain applications off the table, Fortune columnist and author Annie Fisher shares the five top reasons from 1,500 recruiters and hiring managers why they don’t call an applicant.

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 Outdated skills.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” as a red flag.
3. Getting fired. Most recruiters and hiring managers ranked having been fired as “severely damaging” to candidates’ chances of landing a new job, which isn’t too surprising.
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; 28% said the same of someone with any gaps in his or her employment history.

So what can job hunters do to overcome some of these red flags? You can’t change your job history, so be forward thinking. Stay current and active in your field. Take courses, earn new certifications and participate in internships if possible.  Networking at trade shows and other events is also helpful to acquire current industry knowledge to share in an interview. And network with friends, family and former colleagues as they can help make that person-to-person connection to help you get in the door for an interview. Then it’s up to you to sell yourself.