I'm having a similar problem with my two brains.
One is only a few weeks old, one i've had for well over a year.
The only thing different is i had to remove the gfo from my system because it was stripping nutrients. My sps were going pale, my gorgonians weren't extending polyps. Once i removed the gfo and started feeding more, all that reversed. But now my brains don't expand. They were doing great until now.