Clams are also susceptible to poor shipping. With all the things involved in the collection, holding, distribution, shipping, etc, it could have been overstressed and on it's way out before you even bought it.

I had a deresa clam who's shell grow to over 12". When the mantle was extended it, would take up over a 1/3 of the bottom of a 90 gallon tank. It develops a very pretty brown/blue/green spotted pattern along the edges of it's mantle. It was also the most tolerant of all the clams I've had experience with. Very good clam for a beginning aquarist. Here's a pic of mine about 2 years before it died: