There have been recent articles about the bristle worms sheltering under the clams. They do pack a sting, they do eat the old bistel threads and they do eat the mucous. I don't know at what point it becomes a problem for the clam, but, some people have reported bristle worms making a hole at the normal bistel opening at the hinge and entering the clam there. Perhaps feedings, light and other shelter areas are contributors to the clam's overall health that makes the difference between tanks? Ask Felipe (one of our popular lfs owners), he'll tell you the same thing as Jenn and I have. It didn't happen to my clams, but, then I didn't let the worms shelter there and my clams were growing new shell all the time before I sold them all.