Check your pH. if it's under 6.5, that may be the reason. Snails need alkaline waters to survive(or at least have a good shell). Your snails might be old. Snails only live for a year in tropical climates 75+ degrees, and if you got your snails all at the same time, they might all be dying of old age and just looking like there's something wrong with your snails. Copper kills invertibrates and if your clams and snails are dying, copper may be the cause. They also could be dying of starvation if you don't feed them enough. Try putting algae discs in there. You don't need to own snails if you don't have live plants to fuel algae.