Doesn't sound good for the one not moving. I would suggest you remove the non-mover to a small clean bowl of tank water. Make sure you don't bring along any sand or detrius. Leave the clam alone for a hour or so. If he "spits" out crud as evidenced by crud in the bowl, then he is alive. No crud in evidence, then he is dead.
I have 2 in my fuge and they both burrowed right away.
I have read others on the reef forum say you can by fresh clams at the supermarket and follow the same procedure. Put the ones that spit into the tank or fuge, cook the others.