First get rid of the wooden sushi roller if your going to use wood then use something thicker that can soak up more of the moisture the cheese gives off , or you have nothing else put paper towels under the roller I suspect that the sushi roller is keeping the cheese wetter than it likes .
Another possibility though unlikely because you mention it has happened more than once is that some how one side is getting more salt
I also think you can get rid of the the bowl of water if you have a lid for that container , this will also allow you to use a smaller container for those cheeses , and thus more room for cheeses in the cave .