Keep posing. Keep looking at your back between your sets. Close your eyes and imagine your back getting bigger.

The problem as you said is that your mind and muscle connection. According to me back is the most difficult muscle to build even though its such a big muscle. The reason behind it is that the mind cant see it. When u go a lift a dumbell to do one arm it does not know what muscle should it activate so its activates shoulder, bicept etc. So concentration is the key.