Why won't my boyfriend's nipple piercings heal after a year and a half?

My boyfriend had his pierced in October of 2008, and he still gets "crusties" and scabs. However, his nipple piercings have never been infected, according to him. He currently wears 10 gauge barbells, but used to wear 6 gauge horseshoe-shaped barbells. To the best of his knowledge, his rings are all hypo-allergenic stainless steel. What could be causing this? I'd think after this long he wouldn't have any sort of problems. He's normally a very fast healer, and has 14-gauge earrings without problems.