Your clams look pretty good. They dont look sick, and yes, it can be healthy and not open all the way. When I see pictures of Maximas in the wild, they are usually not open all the way.
My gut opinion of your situation is your clams are on the early stages of malnutrition. Everyone talks about all they need is intense lighting, but this simply is not true at all. If you put a healthy clam alone in a tank with all the lighting in the world, it will die of starvation in 3 - 4 months. Thats a fact. I may get some slack for saying this, but I am convinced that if you maintain your reef at zero nitrates your clam will die within 4 months. Usually around the 2 months point it starts retaining its mantle and becoming less responsive. Also know feeding them does not help( directly, anyway). What your clams need most is a nitrogen source (nitrates), and some DOC ( why we dont run carbon anymore) and light. If any one of those factors are not met your clam will die. THose clams are really healthy to start with, so its not too late for a turn around. Feed your fish a little more and lay off the water changes for a while. Get your nitrates to atleast 1 ppm, and dont worry about it with SPS or anything else. If you are confident in your skimmer, stop running carbon and watch your clams get fat and chubby. LOL.