Just a thought, maybe a coincidence...
I noticed a lot of surface 'scum' on my water, like a light sort of oily layer. Turns out I had the water level too high for the filters, etc, to work properly. So I lowered it and the surface it crystal clear now. (the skimmer went mad for a day it two!)
Now my snails are all happily snailing around the rocks and lower levels.
It it possible the bad stuff on the surface was acting as a water-air barrier so the water was not aerating itself as efficiently as it should causing the snails to seek higher ground?
Just a thought.