Why do my hermit crabs die?

I bought 3 electric orange striped hermit crabs (not halloween the rare ones from marhsall islands) online for 35 bucks for 3. Two dissapeared, one of them was in his shell and started acting listeless, next thing I know he molted or his stripes turned white and then next thing I know another brown hermit is in his shell, there is no lack of open shells in my aquairum either and my hermits never die. what is the problem?