Well aside from the shading that has been mentioned, there were a few other areas that you can improve upon. First, the hair in the front view looks very large. Almost too large for her head. And it also seems a bit too squared off. You should make it rounded off a little more. And in general, for the front view, it all looks very stiff.
The side views were done better, but again, the hair seems a bit odd. You have her hair moving over her shoulder when her arm moves back. The hair should move back when her arm is back as if it were being pushed back by her arm. And when her hair does move back, you have an odd black outline around it. Also, on the front view, her hair is parted with large bangs falling over the left side of her face. That should be reflected in the side view. The left side should have that large chunk of hair over part of the face.
The back needs a lot of work as said, but one of the biggest things you need to work on is the hair. It doesn't match the front view at all. It has a different shape, length, movement style and the way it's shaded makes the color look different. Also, hair is never cut perfectly straight like that. Try having it taper down more.