Have you noticed that any of the debris in the tank gets caught in that corner.
There were several of my snails which found a treasure trove of little bits of food and stuff in the tank which got circulated between the pump and circulation and they started hanging out there. I moved a couple of rocks to change up the flow and they stopped going to the pig trough corner!
Try moving the ring for a few days and see if the buffet drying up gets him to move for new ground! I think you have a lazy snail!!!