I have to admit, I have not had much success with clams. I noticed mine not wanting to open so much. I checked the tissue and I want to know if this looks like it has been snacked on.
Look into freshwater dips for pinched mantle. I don't have a ton of experience with clam issues, but I think that's a good place to start. James Fatheree (sp?) has published a number of really good articles on Advanced Aquarist.
I'd recommend doing a freshwater dip using RO water for 30 minutes, just make sure that the PH is the same as your display. In a few days,you should start seeing some results, if not repeat the process.