It's pretty normal for terrestrial hermit crabs to have difficulty molting if conditions are not good enough, I find it odd that a marine hermit would be having bad molts. I don't know of any disease that affects crabs that would do this, though. If you can't get the answer in this forum try posting in in the disease forum, it's worth a shot.
Sounds like it could possibly be a trace element deficiency? Crustaceans need Iodine, which is supposed to come in your salt mix, so I really don't know how it could be reduced unless something is removing trace elements from your system. Do you leave the molts on your tank or remove them shortly after they molt? I've heard it's better to leave the molt in for a while after each molt. I leave mine in and usually can't find any trace of them after a week.