One clam gaping and the other not...

I have 2 maxima clams in my tank and one is healthy and looks good and is growing and the other is not. They're at the exact same level in the tank so they're both getting the same amount of light. The one maxima is gaping and won't stop and I've tried to move him closer and further from the light and nothing seems to be working. Water Params are fine. I just don't understand how one is healthy and thriving and the other is not. and the unhealthy maxima keeps putting himself in the sand. I've tried constantly putting him on rocks but he always moves himself back on the sand bed in the same general area. Need some advice...