Hi Mark,
Well from all the research I've been doing lately and from talking to Kevin at Aquatic Dreams and Barry at Clams Direct, clams don't like a lot of flow. It's okay if their mantle flutters a little now and then but if it's constantly in motion or being blown straight up, it's too much flow. Might be why it's not staying where you put it. Is there anyway to find a different spot for it or maybe move some rocks around to kind of shield that area from flow?
I think a Crocea will be my next clam as well. However, because I'm running T5 lighting, I'll need to put the Crocea higher in the tank. I'll have to do some thinking about flow issues before I attempt that though.