Snails are really sensitive to Ammonia, Nitrites and Nitrates, so major changes in those can cause death.

They also need hard water to live. Hard water contains calcium and they need this to keep their shells hard and strong. Without it, their shells will turn white and begin to errode, killing the snail.

A pH of 7.5 and up is required. (The water softener would be what's doing it, in my opinion.) The softener is taking the calcium out of the water. (It's also not great for fish, as water softened water can sometimes leach minerals out of the fish.) I've got water with a pH of 7.8 - 8.0 and all my fish and inverts are doing great. The shrimp are shedding like mad!

If your water doesn't have enough calcium in it, you can buy a cuttle bone (the type you give to pet birds.) Boil it to make sure it has no germs, then break off a piece and toss it in the tank. It will dissolve into the water. (I've also heard of people putting crushed sea shells into nylon stockings and hiding that in the filter. This also helps put calcium into the water. Just be sure the shells are free of salt as salt will kill the snails.)

I hope this helps! If there is anything else I can do to help, please let me know :)