So I've got a 55 gallon tank and wanted a snail so I bought a little zebra a while back and dropped him in. He never even came out of his shell then he died.
So I invested in another one. Same result. I thought "There seems to be a pattern here."
So I went to the Pet Kingdom pet store here in San Diego to consult on what might be wrong because if those guys don't know it, it isn't located in the sum total knowledge in the civilized world.
Anyway, the guy told me snails can be very sensitive to changes in pH and it might be the change in pH was shocking the snail. So here's how he said to fix it:
You open the bag with the new snail and his water.
Take a 5 ft length of air tube and start a siphon from the snail's new home to him and his bag of old water (but not into the snails water yet) Just get the water going - I suggest another bucket of some kind.
Crimp the air hose near the top once the flow gets going. Transfer the hose from the starter bucket to the snail's bucket.
Then use it as an IV drip and literally do a tiny squirt of water out of the hose into the snail bucket every 2 or 3 minutes for an hour - very tiny. Yes, you will be bored. Bring some reading material.
After that I let him sit in the bucket for another hour. Then in he goes.
He stayed in his shell for about two weeks (I hear this is normal but also one of my Africans likes to push things around the tank and the snail was just too irresistible to him, I bet). The little snail is doing very well now and has the largest following in the office after everyone witnessed the whole IV thing - everyone was pulling for him and they all check on him every few days.
If you're having snail problems introducing them to a new community, you might try the IV.