Are they dying? be predated upon? or just starving to death? I guess what I mean is what do you mean?
In my reef, for example, 75 snails became maybe 45 snails due to lack of algae (starving), hermit predation (argh), flipping upsidedown (darn astreas). Now I am noticing small (baby) snail starting to pop up. In any case the tank achieved a "CuC balance" after that culling phase at the start.
If you are losing them becuase of external things (accidental copper introduction, oops, or interceptor treatment, etc...) then that is another story.
Just my opinion--landlord