Spanish Dancer, I have not been able to keep snails for very long. I gave up. I've kept shrimp, stars, crabs, hermits, urchins and they lived. (of course it didn't help that we had a cc star that liked to knock them off the glass).
What size is your tank? Do they have enough to eat? Are they on the glass? if any are look at them very closely, to see if they are moving, but staying in one place. We had some bubble bees (I think anyway) and they didn't really move.
What are your other levels?