I don't think the answer is too complicated really. We have an overall lack of quality in the forward and attacking midfield areas; only Robinson (on a good day) looks like an above average championship player.
You then add to this a lack of confidence, which is getting worse as the goal drought goes on, and you have what we have, an inability to score.
I suppose that a lack of tactical nouse, set pattern, structure may also be part of the problem though.