Snails dying can be from a few different causes. Starting with the most common.
1 Hermit crabs looking for new homes. Hermits are also known to kill snails just for food.
2 Lack of food. Competition for food can create issues where either many will die due to lack of a food source or unable to find a food source, or they cannot compete with others for that food source.
3 normal life cycle. Some sanils can and do live years. There are also some snails that are from colder climates that will live for short periods of time in a tropical water tank, but due to the increase in water temp their metabolic rates are increase which shortens their life cycle.
4 Chemicals in the tank and or water. Chemicals such as copper and others that inverts cannot handle. I do not believe that this is your problem because you said that you have shrimp in the tank, which would also be effected by these chemicals.
5 Predation by fish. Some fish, especially triggers and puffers, love es car go
I hope that this helps
Rich