Be sure they are not just simpley molts. After being placed in a new environment, it is quite common for most crustaceans to molt. Most small hermits are also quite adept at hiding and cannot often be found when they don't want to be.
Do you have any other predator fish/inverts and where they all moving about after introduction to the tank?
If you are absolutely sure they are dead, then I might suspect how they where acclimated or possibley something amiss with the water specs. What is your source water ?
Cheers
Steve