Hiya Clownfish =)
Snails are, of course, very sensitive to copper. But since your other inverts are fine, you can rule that out. The only other possiblities I can offer would be poor acclimation or possible starvation.
Snails need acclimation time to adjust to changes in salinity and temperature. Dr Ron's advice is, "Remove them from the bag. Let them dry out in a cup or bowl for about 15 minutes. Then take them one by one, and place them in the tank at the water line with the aperture next to the wall. The animals will extend, fasten to the wall and move into the water of their own accord. In effect, they will self-acclimate."
This works very well I think. Its easy on the snails AND me!
I don't know if you are looking to try different snails in the future, but I thought I'd add this little list.
: Great detrivores and sand-stirrers. They will hide in the sand until feeding time.
: A great algae eater, but sometimes fall off the glass onto their backs and will require a little help to right themselves.
: Algae eating machines. Ideal size, won't knock over corals/rock, and can move very fast.
: Will breed in your tank! You will only need to get 4 or 5. They're great algae eaters and stay very small. I give these to all my friends. =)
Will also breed in your tank. Good algae eaters and sand-stirrers.
: Little vacuum cleaners. One per 2 sq. ft. of sand. They'll eat diatoms and, sometimes, cyano
: They are big and clumsy. They'll knock over or move anything that isn't glued/nailed down.
Red Moon Snails
: Cold water species.