FORTUNE — As a senior recruiter at Waltham, Mass.-based search firm WinterWyman, the most common complaint Cesar Ulloa hears from irritated job seekers is that headhunters seem to be ignoring their phone calls. Frustrating, of course, to hear nothing — but the reason, according to Ulloa, is almost always one of these five:

1. An unpolished resume. “In addition to showcasing your background, experience, and education, your resume can demonstrate your attention to detail and even your intelligence,” Ulloa says. He’s seen plenty of CVs with spelling mistakes, typos, missing information, or hard-to-follow formats.

“Be sure to have someone proofread your resume and critique it for format and content,” he suggests. Simple as it sounds, skipping this one step can cost you a meeting with a client company where you’d like to work — and most headhunters won’t be in a hurry to tell you so.

2. Gaps in your work history. Even if you have terrific experience and skills, “a long period of unemployment can make it difficult for a recruiter to place you, since hiring managers typically prefer candidates who are employed,” notes Ulloa. “But sometimes being between jobs is unavoidable.”

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If that’s your situation, he recommends lining up some contract work, including short-term projects: “It’s an effective way to keep yourself in the game, polish your skills, and meet new contacts.” It will also make the recruiter’s job easier.

3. Missing skills. “Many employers have specific requirements for the degrees, skills, and certifications they want in new hires,” usually spelled out in the formal job description, Ulloa notes. Yet, in some resumes, the relevant information is buried or not mentioned at all.

He urges job seekers to “be sure that all your talents are clearly listed on your resume. Repeat and highlight, in your cover letter, the ones that relate particularly to this role.” If a recruiter has to explain to clients how exactly you would fit into their job opening, he is probably not seeing the kind of enthusiasm that he can’t wait to tell you about.

4. Inconsistencies in your background. Now that hiring managers can look you up on different social media sites, and perhaps find your resume on job boards like Monster and CareerBuilder too, “consistency is critical,” Ulloa says. “If your LinkedIn profile shows different titles or dates than your resume, for example, it will create misgivings in the recruiter’s mind, and it could be a reason you aren’t called back.” Make sure that, everywhere in cyberspace where your name may show up, each detail attached to it matches up with your current resume.

5. There’s nothing to report. “If a recruiter called you and then hasn’t reached out again after an initial conversation, it’s okay to follow up,” says Ulloa. “But at most, call every few weeks.” Finding the right fit between client company and candidate can take months, and Ulloa avers that recruiters rarely, if ever, overlook qualified applicants or forget to call with important news.

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“If there’s a fit, a next step, or a question, you will get a call,” he says. It may take longer than you’d like, but Ulloa counsels patience. Constantly calling or emailing the recruiter often gets an otherwise promising candidate written off as “overly aggressive,” he says — which may mean the recruiter will never call back at all.