[James From Cali]

In my experience it is when the water qulaity is poor they leave. How iI found this out I put about a hundred MTS in a 10g tank. Fed 2 a week a few algae tabs and did no water changes. In another tank I did the same except I did water changes. In the non changed tank the ammonia rose to above 5 and I had about 50 die off, but still there were many babies, and the rest were on the glass during day time. In the other tank the ammonia spiked to .5 at the most and no snails were out during the day(minus the few that would be shoved out of the gravel). So this can be a sign of water quality. I would test and do a water change and wait to see if this continues. It may also mean it is time to attempt to thin out some snails.

As far as your snails, if their shells are white it may be calcium deficiency and they are trying to find a source!?!?!?!? It really all depends.