When my biceps finally did begin to respond, they grew because I was actually feeling the weight. As I became more advanced in my training and had logged some real time in the gym, they stopped growing at the same rate - almost as if I was over training them again. My emphasis on heavy back movements was straining my biceps and causing them to be pre-exhausted. I cut back on my arm workouts, and they started growing again. In my opinion, over training the biceps is more damaging than almost any other physique indiscretion.
Biceps are easy to over train because they are among the smallest body parts to begin with.
Moreover, they participate in the movement of various other body parts. As they are a necessary part of back exercises, they tend to become exhausted easily. A second workout for arms alone is often too much for such a small bundle of muscle. Over training the biceps can slow down progress in the entire upper body. When the biceps are tired, they can't be fully available for heavy back movements.