People seem to have varying luck with them. I was not one of the fortunate ones :( I had 4 small maxima's about 2 inches long and all of them died. I only have one crocera clam now who is still doing good, but he's like 4" across. Some things i've learned about them:
1. Cowries are NOT clam safe. I had a larger cowrie snail and it knocked one over and proceeded to eat it. LAME
2. If they fall over for some reason, pick them up right away. One fell over and I think and died for that reason (didn't see it fast enough).
3. Thrive sells a clam holder kit, don't bother with it, they won't attach if they are smaller maximas. You are doing better as yours attached to the rock already.
4. You need to spot feed them once in a while with Phyto plankton (can also area feed in the water colum but makes your water messy faster). It has to be very small sized. I'm forgetting, but smaller micron size the better. They can't live off of just light when they are small. Once they hit the 3.5-4" mark they can rely on photosynthesis more than feeding.
Good luck, I hope yours does great!