I read a post Maldorf and several others saying that their gains stopped after the 8th week or so if, I remember correctly. Same here. I give it 2 more weeks to solidify my gains, adjusting food (protein mostly) upward a little and then come off. IMO, why waste time and gear for nothing?
Why does it plateau? Some say SHBG rises through the 20th week of a cycle, then corrects after that. Could be that you're gaining weight, but still eating like you were 15 lbs ago and your body needs more to support more muscle.
Its gonna be dose or diet or overtraining before that, I think. Have you taken a de-load week or two, training lighter for a couple weeks? That works WONDERS for me.