Ross, I apologize if you mentioned this info already, but, what do you keep your salinity/SG at, and just for curiousity what brand of salt do you use? How often are the water changes at the moment and how big are they? Oh and what are you using to measure your SG?
Not promising the answers to these questions can give me an "aha" type moment, but it's worth a try..
I'm rather stuck on what you said about the crustaceans having trouble molting. This seems unusual to me. One time, I can pass off as an anomaly but not when all are having problems. Maybe there is too much iodine in the water, or not enough (iodine is apparently a crucial element needed for triggering molting). It sounds like they might be molting prematurely, or perhaps molting too late somehow.