Firstly, have you checked your water parameters? Snails are especially sensitive to ammonia. Also, please post the pH of your tank water and tap water.
I have naturally soft water and have always had problems keeping snails alive. The pH in my planted tanks containing driftwood drops to about 6.0 between water changes. The fish don't mind it a bit, but the low mineral content and low pH prevent the snails from building a strong shell. The operculum (the snail's trapdoor) is especially deformed.
Recently, I've had great success keeping snails by removing all driftwood from their tank, keeping plants to a minimum, buffering the water with crushed clamshells, and dosing the tank with 1/4 teaspoon per 10 gal Kent Marine calcium at every water change.