Afraid Mark hit the nail square on the head, sometimes they just die.
Sometimes there's an obvious reason, many times not.
That's not to say that sometimes they look pretty rough. If you get a spawn event for example they can look pretty rough but then snap back.
But, for whatever reason, one thing I've found to be a terrible thing for a clam in a tank, is for another clam to die in that tank. It can very often lead to a cascading of clam deaths - whether it be from them spiking the tank or something else, I don't know. But I guess what I'm saying is, while on the one hand "don't give up until you're 100% sure" the flip side is if you are close enough to 100% sure that there's no recovery, and you have other clams, you may need to think about pulling it out.