I think it would help to further simplify and refine the shadow shapes. You have a good start here. Go a bit darker in value and follow the edges of your mannequin. Having these shapes specified as the areas in shadow will help you fill in your mid-tone values.

The head certainly has a sense of three-dimensionality to it! And some cool effects your getting with that pencil! I guess my question would be is if you are using any construction methods for the head? The most famous is the Loomis method but there are others out there to be explored if that one isn't giving you the desired outcome. Hope you keep at it, I spent a year drawing cubes and it made me take an entire year off from drawing T_T I do regret that decision.