There seems to be a drawing problem with the mouth and chin area that is distracting me from other considerations. The typical human proportion is to have the center line of the mouth (the line between the lips) above the halfway point between the nose and chin. That is, the mouth is not equidistant between the base of the nose and the underside of the chin, it should be quite a bit higher than that.
Also, I agree with Alexandra's comment about the image having a feeling of flatness about it. I think that's mostly because of how the model was lit. The source of light seems to be coming from almost directly behind the viewer/artist. With that kind of lighting, you will get a flat image. For a more three dimensional feeling the lighting should be coming from a bit more to the side.
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