Maybe it's the maturity of the tank? I think so too but not sure, I didn't start to add sps until a year latter, after the tank was set up and I started with digi's.
Maybe not enough disolved organics?
I think you might have a aussie red fire stick (as I know it) if so I have one too it was the only sps not growing. I started to feed my fish more and have noticed better growth. I have a PM skimmer rated for 250 gal. and a aquac rated for 120gal on a 90 gal tank.
your tank might be runing to clean
I know some one else that bought a frag of the same colony and his grew twice as big as mine and he wasn't using a skimmer. That's the reason I started to feed more.