NEW YORK (CNNMoney.com) -- Despite millions of unemployed job seekers desperate for work, many open positions are languishing unfilled. The reason? Not enough candidates.

With job openings largely concentrated in specialized industries like health care, green technology and energy, some employers say the problem is finding qualified workers, which are in short supply. Meanwhile, they are inundated with eager candidates from other industries who lack the skills and experience that the job requires.

According to a recent survey by Human Capital Institute and TheLadders, more than half of employers said "quality of candidates" or "availability of candidates" are their greatest challenges -- despite the recession.

Mary Willoughby, the director of human resources at the Center for Disability Rights in Rochester, New York, has been trying to hire registered nurses, home health aides and service coordinators for several of the agencies that she oversees.

Many of the positions, which require specific skills and offer salaries in the range of $30,000 to $45,000, have been vacant for six months or longer.

The job postings, which appear on CareerBuilder, Craigslist and some regional sites, garner a lot of attention, she says. "We get tons of résumés from people. We are just not getting highly qualified candidates."

The problem, according to Willoughby, is that they are bombarded by résumés from job seekers without the two years or more of health care experience necessary. "We're seeing a lot of people trying to break into the health care arena," she said.

As a result, human resources spends too much time sifting through résumés for people who aren't remotely qualified, and can't find many that are. "We've gotten close to 300 résumés for a service coordinator position. Out of that we brought in four people," she said.

Those that didn't make the cut included someone with previous experience as an office clerk and a job applicant with a bachelor's in mathematics, currently employed at a café.

Willoughby recently instituted a hiring incentive program to encourage existing employees to refer viable candidates. Those responsible for bringing in new hires are eligible to receive $2,500 to $5,000, depending on the position. She has also added in a signing bonus for the new employees.

Things are even worse on the higher end of the pay scale. At wireless leasing firm, Unison Site, a position for director of lead generation, which pays $90,000-$140,000, has been open for three months, with no candidates in sight.

"With the job market the way it is, we should be able to recruit really good people and it hasn't worked quite as well as we wanted," said Joe Songer, co-founder and chief financial officer. "My problem is when I put an ad out I just get bombarded with people that aren't qualified."

Typically, the jobs that are the hardest to fill are those that require unique or extensive work experience, according to management professor Peter Cappelli of the University of Pennsylvania's Wharton School of Business.

For job seekers, applying to those types of positions may be worth the off chance that one responds with a request for an interview. "They think, I've got nothing to lose," Cappelli said.

Recruiters recommend that job seekers create a targeted list of companies with a clear match to their background and tailor their experience to the job they are applying for, rather than blanketing all available job openings with the same résumé.

"Eighty percent of jobs are being obtained on personal referrals so candidates that are spending the bulk of their time sending their resume out blindly are not being the most fruitful," said Carolyn Thompson, president of CMCS, a boutique staffing firm near Washington, D.C.

Thompson advises job seekers to network within those target companies, whether in person or through social networking sites.

Without a contact at the company, résumés should highlight and emphasize any relevant experience specific to the job opening, added Jennifer Becker, market director for Ajilon Professional Staffing. "You really want your résumé to very quickly and easily reflect your relevant skills and the value you can bring to the position."

"If the client has to look for it, you are probably going to get passed over."