AGreef,
Try placing a flat piece of live rock under your sand bed and then place the clam on top. Clams like to attach their foot on a solid object. If they cant do this, they can become very stressed.
How large is the clam? I had one that was less than an inch. He was unable to attach his foot, became stressed and died. I later obtained one 3 inches in length. Within a couple of days he attached his foot and has been there since last winter. -Good Luck
Also, here's a grainy pick from my palm.