They undoubtedly smash the snail/hermit shells to pieces in order to get the food out. They will use their hammer/claw (depending on the species) to crack the shells which usually sends it flying across the tank. Look for a pile of smashed shells, or wait a couple hours after lights out and get a flashlight on the tank... be patient and watch. Oh and make sure it's not sand stuck in some impeller, or something mechanical, just in case.