Astrea snail question/problem????

[Canadian Man]

So As of the past few weeks a big majority of my snails in my tank dont seem to stay upright.
So I flip them all back over again and put some back on the rock work.

Well a few hours later they are all cringed up?? :confused: and either lying upside down on the sand bed or right side up on the rock work but their oval shaped foot is all cinched together.

Sorry I can't explaine it any bette than that but does anyone have any ideas? Please?