Check the water quality as Hara has suggested to be sure, but "dirt stupid" really does sum up Astrea snails to a "T".. LOL
Here's a list of snails you should look into that will be of much more benefit providing all the water specs check out.
great for detritus and the sand bed.
especially good at film algaes..
cleans both the glass and the sand bed..
cleans up the uneaten food bits as well as detritus... looks cool too.
another excellent algae eater..