I would say there is a very good chance your tank has stay current that is affecting the snails. Especially if they aren't moving much after you first put them in.
Do you have any koraila powerheads? If you do they are likely the culprit. Heaters is the next likely thing but it could be anything. I actually crashed my tank adding more and more snails until I figured out the problem. I only found out when I got a shock. If you don't want to test with your hand get a volt meter, otherwise just cut your hand and stick it in the water and see if you get a shock
A grounding probe will help but not prevent this.