I find that if they don't get enough to eat, they often end up turned over from falling off of the rock work. If your rock work is clean, the snails might not be getting enough food. You might think of bumping up your lighting some to grow algae faster or increasing your nutrients: phosphates and nitrates, just a bit.

Can water snails turn themselves over?

Being upside-down for short periods of time won't kill the snails, but it does leave them vulnerable to predation from tankmates, and they can't feed or do any other normal snail things. Some snails can flip themselves over like acrobats, but others may need a hand if they get stuck.

How do I know if my aquatic snail is happy?

A happy snail will move all over the tank, up and down the sides, over rocks and plants. Unhappy snails stay at the top edge of the water, and even try to crawl out of the tank.