Found the problem. It is the copperband picking at him.
I put a piece of transparent mesh attached to acrylic rods above it, like a tent, so the copperband couldn't get to it and the clam opened back up to normal. Removed it for a couple days, closed up again. When added back, it's happy yet again.
Such a deliema since I can't bring myself to part with either. Maybe the tent will need to stay....