Have you heard of taking the snails from bag and placing them against the glass at -or just above- the waterline? I've seen this attributed to Dr Ron Shimek (not verified), and the snails supposedly take up to an hour or two to "acclimate themselves" - like some of us do at the beach.
I tried it with a couple, and they were fine.
<< PLEASE SEE reefburnaby's comment below (Feb 14) - his experience speaks loudly. >>