possible multiple molting crisis!!!

[Shannon_LHC]

hello I'm new to this site and owning crabs…. we let my son bring some home from a shop in myrtle beach… after one died i started researching and have found out all the stuff required to keep them alive… i have put together a home i think is suitable based on my research and yet I'm having problems with them still… the most pressing one is the biggest crab we have is not in his shell…. i changed the substrate and added some stuff to the tank yesterday, he seemed very happy exploring (which prior to this he had been sitting by the heater not moving much) i noticed him in the salt water bowl very streched out looking as if he was going to come out (he changed his shell a day after we got him so i thought that is what he must be doing) this morning though his shell is in the salt water bowl and he is in the corner of the tank…. lifeless... he might be dead? or could he be surface molting without his shell??? it is not too hot in the tank, in fact i think i might need another heater i have a thermometer on either end but they both read the same temp of about 71 (this crab I'm speaking of also had a little salt water bath before i put him back in the tank yesterday after changing out the substrate) another crab i have (which was bought from petco) is missing… (as of yesterday after substrate was changed) i don't think there is anyway he could have gotten out so i thought maybe he buried himself… wouldn't i see a hole or some signs of digging if he was under ground though? my third crab (also from petco/as replacement to the boardwalk one that died) is the most happy and active of them all but since substrate was changed yesterday he has been hiding in his log and hasn't come out) lastly (another one from the boardwalk) has been sitting in the same spot of the tank (pre substrate change) for a few days before i checked him, he was def alive so i put him back… yesterday when i changed substrate i took him out and put him back where he had previously been sitting… he is still alive but still not moving…. he is very tiny I've read about the surface molting so i guess thats what he could be doing? i haven't isolated him and nobody seems to be messing with him. please help??? what is going on??? I'm trying very hard to keep all these little guys alive and happy.