The whole piece is too light. While looking at it relatively (comparing say, the hand to the hair) the values seem right but converting this to a colour image would yield extremely light skin. As storm had said, darken the shadows and the hair.
Darkening those however, as a consequence, makes the skin look too pale. As much as it is about value, it's also about contrast. Lighter values make darker values look more prominent and vice versa. Go over the whole thing with a very (I do mean very) light grey and build upon it.
Couple of areas that could use shadows, the area under the eye brow, this shadow is a result of the eye socket being recessed. The ears will have a shadow due to the hair. There's also a white spot on the right hand side (our perspective) that needs to be filled in. The original picture is also extremely dramatic since the hair is casting a very prominent shadow on her right eye.
This isn't meant to bash your work, this is just a critique, the works looks solid thus far