You have a good instinct for tone. Hair is mostly about sections of tone with soft edges, and a suggestion of texture rather than the individual hairs, which I think you've done. Think about where the highlights are in the hair - where was the light coming from? In her hair it seems to be directly in front - but her face suggests the light was coming down because the highlights are on her forehead, top of nose, chin... so... the highlights in the hair should match the direction of the light on her face.
