Is the clam attached to anything or just sitting on the sand?

Bristleworms won't attack it, but may be irritating it. They'll hide under the clam the same way they would under a rock, so if the clam is unattached it could be bothered by their presence. Try to get the clam to attach to something.

That may not be why the clam isn't opening though. There are lots of possible causes.

Have you checked it for pyramid snails?
What kind of lighting do you have?
What kind of clam is it?
How long have you had it?
What fish are in the tank?
Is the mantle looking pinched or withdrawn?