I hope I'm not stating the obvious here, but could they be killing each other while fighting over shells? I put some empty shells in my tank to try to prevent that from happening.
Also, in response to lion_crazz's comment about the drip acclimation time, I'd been told by several LFS that they only need to drip for a short time . . . 30 minutes to an hour. That is all I've done with mine and luckily haven't lost one yet. Should they really be dripped up to 2 hours?
Thanks,
Mike