IME when a clam dies in a couple of days, unless it's a very small 1" clam, it's generally something in the tank has a taste for clam even if you don't notice it, or the clam just wasn't very healthy to start with which seems to be more often than not and shipping or new tank stress does it in. What size were they? How did they end up going? That is extended fully then dead or progression of slowly closed? Did they have white growth around the shell when you bought them (most important).
tank is 300 gal and been up for 7 yrs. clams were 2.25-2.5" from Liveaquaria. they all were open normally for few days and then one morning you can see mantle shrinking losing grip and within a day or 2 mantle is all gone. clams were clean on the shell