Have you had the shrimp very long? When i first get new shrimp they tend to molt shortly after I get them. It's their way of adjusting to the new water.
I sure hope that's what my little guys are doing. Got them last Wednesday (12/16) in an RAOK deal. They've been out and about and running around happily until the last couple of days. Yesterday I saw them both (in the 10g) just once, and today just one and just for a minute.
I posted a question about normal molting scheduules for RCS yesterday and didn't get any answers. Can anybody who has raised them tell us how often they usually molt while growing up? I'm assuming/hoping they're hiding out under the rocks for some reason, like either part of the normal growth process or they're sulking about someone treating them rudely or snubbing them at a party or something.