What's wrong with my mystery snail?

About a week ago he withdrew into his shell, and he hasn't come fully back out since. He is laying on his back, with the opening of his shell facing up. After he remained immobile for five days, I reached into the tank and flipped him over, thinking he was stuck. A couple hours later I looked and he was about halfway out of his shell, so I got excited. But a few hours after that, he was laying on his back again. Ever since then he has been coming out of his shell, just the tiniest bit, and then going back in, so I know he's not dead. I don't think there's anything wrong in my tank. My betta fish is in the same tank, and I've had him for three years, and he's still in great health. Is my snail sick or something?