Help! i need help with w my 2 hermit ....?

i have 2 hermit . one small about the size of a nickel/quarter and one the size of a half dollar. i have several shells and i've noticed that they are always switching shells. i thought i have the right size for them to move up to but maybe not... why are they switching shells several times in a day? and i think that i've screwed up the substrate. they aren't very active and i'm not sure what to make the substrate out of. i have a coconut mat with sand... but after getting the coconut mat wet, it doesn't appear that they can do much digging in it! help!