How long has your tank been up and running and what kind of lighting are you running? Also what are your water parameters? Clams can be tricky because when they start to go its often too late. In general they need an established, stable tank with strong lighting. As for what type look at Maximas over 2.5" or Deresas.
I would stay away from Maximas. I would get a deresas or a crocea clam. My 2 crocea clams survived 4 years 4 tank moves and 2 tank crashed. But like Reef trend says the key is getting a healthy one. Once it goes down hill it?s over