[Poggio]

You need to pick a light source and then determine where all of the shadows fall. Right now they are just arbitrary due to the curves of the muscles and that is fine because that is expected, but places like the hat for example doesn't make sense. If there is a highlight on the right side then there shouldn't be that deep of a shadow on the left where it folds and more of a shadow on the left side where it curves around if that makes sense. With out that sense of direction the image still appears flat.

Edit: Places like her left arm and well left side in general would have more of a shadow as well. Over all though I think its a nice drawing nice proportions and pose. The figure may be a bit hard to tackle at first for reference to see what I mean I would look at an object with a light on it from some distance, like a fruit with a lamp to the right or left, not directly but to the side.