How long have you had them and what fish are in the tank? Sometimes shrimp go into hiding until they feel it is safe enough to come out and nobody is going to eat them. And if are about no bigger then a fingernail, then you just might have to wait a bit longer. You should still be able to see them, and if you find an empty shell in the tank, then chances are they are still alive, but hiding to make sure nobody eats them while they are waiting for their new shell to harden. How big is your tank by the way.