sounds like he's not getting enough calcium in his normal diet. I find it hard to keep my bamboos as well fed as I'd like, especially knowing that their preferred foot is baby brine shrimp and daphnia... crushing crab pellets into powder and getting it into the water flow might help him recover after a moult. I often supplement their diet with spirulina powder and crushed blood worm in the water stream, but it really doesn't do what first day hatch brine shrimp do.. they go nuts.
I only say this because, mine never eat anything off the substrate, or their shells. They strictly filter.