They eat and grow for many years.Your snails maybe dying from starvation.
Try feeding them dried green seaweed at night ,you might have to put them on it to get them to feed at first.
Worms can burrow ontop of the shell causing water to get inside killing the animal in the process but i don't think that is the case here.
+2. I got some a few years ago for a hair algae problem. They grew big and fat and knocked over corals left and right. I then moved them to my sump where they couldn't knock stuff over. I fed them nori till I finally gave them away. I had them about 2? years.