If they're not eating the wafers, they're getting their food from the tank. Only give wafers as a supplement if they're caught up on the natural algae. How can you tell? In my tanks, they start constantly patrolling, searching for food. It's different from their normal moving about...it looks like a more urgent, faster search. Even under normal circumstances I won't drop an age tab more than maybe once every two weeks.
Mystery snails are another story. In my experience, they are on the gluttonous side and will vacuum up everything put down, leaving the nerites to do their job, which is cleaning. So it balances out. But I still don't give them an algae tab more than once every two weeks, if that. Those pampered mysteries need to get some exercise hunting for food!
If you put down an algae tab and it's not gone in 24 hours, no one needs it. I would vacuum it out with the weekly cleaning.