HCA: Hermit Crab Association

I have 2 hermit crabs that I got a few weeks ago, and they seemed to be doing fine, but I haven't seen them in days, nor do I think anything in their tank has moved around. The most recent change was I added a bunch of new shells, and the night before then was the last tine I saw them. I got the she...

I've just gotten 2 hermit crabs for the first time, I've had them for a week now. And one of them keeps changing shells (lots of times in the one day), and also just picking up shells and poking his head inside without moving in. Any idea what he's doing/why he's doing it?