So the face on this is gorgeous, first off. I think the lips could possibly use a bit more detail but I'd have to see the reference you used to be sure. The nose and eye and cheeks are all VERY well rendered and you captured the character well - I could instantly tell this was Emma Watson from the Bling Ring.
I think, though, that you maybe got so wrapped up in the face that you haven't given the rest of this the same attention. The shading and highlights on the top part of the hair is nice but the brush you're using is too chunky and your strokes not long enough, giving her hairstyle a choppy feeling when it should be long and flowing. On the shoulder it looks like the hair shouldn't be over the shoulder even though it is. I think part of the problem is shadows and highlights. Her hair has layers but only really on the top - I don't think you need as many fly-aways as you have or you need to at least use a small brush to show the fly-aways are individual strands and not whole hunks of hair. Really, you just need more strokes in the hair, I think. Right now it's got great movement but it doesn't fall organically and just looks chunky.
I also think you should probably crop this. The bottom looks very unfinished right now and the dress looks a bit more like a scarf because I can't see where it should curve around (I think if you crop it elaborating on that may not be necessary). The shading on her boob doesn't look quite right, either . . . it has a weird depth to it. Clearly you have an eye for detail, though, considering the face and neck so I think you just need to do some finishing on it or just crop off most of it and say screw it.
Honestly I think most of the problems here are with finishing more than you lack the eye or ability to fix them. As I said, the face is gorgeous and the arm is well-rendered, too, I just think you need some more time on the hair. If you don't want to do the body just crop it out, you only need the top of the shoulder, right? The background, though plain, is working well with this and I love the light beams across her face. You've definitely captured the look and mood of the character. Good work but keep going!