Sorry for late response. Blame my internet.
Firstly,it's her face. I think you are wise enough to know what's wrong with that(and since HonooNoKarite has said something about it).
But that's not the only thing I found wrong. Again I felt that her expression is not strong enough. I think I have said this in your previous work,in this case,her face is 'empty'. I really can't make sure what emotion is she putting on. Try putting a more distinct expression. Try to play a bit with eyebrow,eye shape and mouth shape.
Secondly,her cannon tube. The double glistening dot you put on the cannon tube makes me think that she was holding a container filled with some kind of liquid. What I wanted to say is it doesn't look like it's a metallic,shining tube.
I also find it really hard to ignore that a significant portion of the outlines are 'not confident' and straight.
Also, try reviewing her right leg.
But still, the design and the choice of colour is great.
Sorry in advance if any of my comment offended you.