Simply put: if a qualified person isn't getting so much as nibbles from internal HR recruiters, that person is not as good as their peers on paper. It's not even the feeling of "always a bridesmaid, never a bride," because nobody even invited the person to the wedding.
The secret isn't a secret though: the candidate lacks something.
Usually the experience isn't as high caliber as other applicants, which brings up the classic conundrum: how do you get (good) experience, if nobody will give it to you?
Micro-employment is an emerging industry where candidates can get that experience, and where some brand-name companies are beginning to take the chance on the "borderline" candidates. Meaning, candidates who are missing just one or two pieces that their peers have, and that is the only tangible difference between the A candidates and the A’s-to-be.
Ambitionary.com is the best thing since sliced bread.