Hi,
I realize this is an old thread but wanted to reply because I'm repotting my brassias today
I have Br. Rex, along with two species brassias I have severely neglected. However, I've grown them for years and my experience is that they want lots of fertilizer in the growing season (spring summer). They also grow faster with lots of heat, again, spring and summer. I stop fertilizing in late fall and winter, then back again in the spring.
Anyway, just wanted to throw my two bits in.