I wouldn't worry to much! Clams can move on their own if they are uncomfortable. My maxi moves! Anywhere I put it, it always manages to come back to the bottom middle of the bow in my bow front up against the glass.lol I've tried burying a flat rock, clam shell in the sand under it, and it never even attempted to attach itself. I've put it up in the rocks, and it just jumps off back down to the sand. So now that is where it stays. This is where there is the least light without being in total shade.
Perhaps your's is getting to much light, and therefor it has turned sideways in order to deal with it.:scratchch Just thinking cause I have no answer for you.lol
Are there any fish that could be knocking it over on it's side?
BTW, how long have you had the clam?