Could his shell be stuck?

Question
Hi,
 I'm a first time crab owner and I've only had my crabs for a few weeks. My large crab came to me (he was a gift) wearing a medium sized shell. I know crabs pick their own shells and only change when they want to but this shell is so small! he always has all of his legs hanging out and when he is (or should be frightened) he cant retract into his shell at all. Because of this he tends to sleep in his salt water bowl. I spray him with myst once a day to make sure he wont get too dehydrated. He is clearly uncomfortable but I cant understand why he wont shell shop. I have 3 large shells for him. 2 D's and an O.
I have 2 other crabs in with him. A small and a medium (the medium has changed shells once to my supprise). I don't know if it makes a difference but all of my crabs are Australian.
In short. Could my crab be stuck in his tiny shell? and if so how do I help him out?

Thanks for your time :)

Braze

Answer
Make sure you've got a salt and a fresh water pool that are deep enough for him to get his shell opening into. Other than that, it's pretty much up to the crab. I have run across one or two stuck crabs in my day, however, much much much more often when I *think* a crab is stuck I walk by later and there he goes, scooting across the tank and into a new shell!