I think you need to get more light on the left side of her face. You shaded the side of her nose that is recieving light. Try to paint what you see, rather than what you expect to see. It's not easy, but you can train yourself to do it. I did a paintover to show you what I mean. Didn't quite get the liknesses myself, but I think they are more believable as real faces:
http://sketchfu.com/drawing/33758
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http://sketchfu.com/drawing/33758