I have a similar situation in my tank. Acans, chalice, cyphastrea, favias all thrive for a few months then quickly lose flesh and die within weeks. However, euphyllia, blastos, mushrooms, montipora do very well. I had a goni that was one of the healthiest corals in the tank, doubled in size, then all of a sudden started bailing polyps out and died within a week. Another goni that always struggled, was never extended, had algae growing on it, all of a sudden came back from the dead and has doubled in size. I can't figure it out and have been frustrated to the point of giving up on LPS. Considering switching gears to either a softy tank or montipora dominant tank.