You have a really nice style and a great vision, and I think your self-diagnosis is accurate. I can see there's a lot of hesitance and uncertainty in your shadow forms. There is a very strong shadow under the chin in the left drawing, but the way the rest of the face is shaded, while not wrong by any means, doesn't follow a rule you've unintentionally set by adding that one deep shadow there. That rule being, if there is a strong, deep shadow under the chin, there must be a bright light coming from somewhere above to create it. What I did here was add a multiply layer to add the shading on the left drawing. If you're worried about ruining your piece, either make a copy of the file or experiment on a new layer. Digital allows us a lot of freedom, so make the best of it! For future study I suggest playing around with some 3d head lighting reference tools, they can be found easily through google search and usually run in your browser. Move the light sources around and see how the shadows on the head respond. Keep at it! | Artwod Feedback