What's wrong with my Malaysian Trumpet Snail?

I've had him for several months in a community tank, but now he's taken to laying on his 'back' with the meaty part up away from the gravel. Also, he seems to be hanging out entirely. I thought he was stuck on his back so I've flipped him over several times, but he keeps going back to this position. I have 3 other snails in the same tank and they're fine, what gives?