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.
uhuhh~~ 0_o dont u see its shy expression?? and yeah~ i have said it in others comment before (in artline version) i only draw using pencil directly (no ruler) , no sketch~ then scan and colour~ just did slightly change with the gun~
pss~ can u download and upload back this picture with mark all the mistake detaily so i can see clearly what is my mistake~~
In all honesty,I'm sorry but It's hard for me to see her in expression of shy. This can be fixed however if you adjusted her eyebrows and the position of her pupil. You don't look straight when you are embarrassed right? Or maybe by increasing the intensity of her blush?
It's very respectable of you wanting to know what mistake did you make. I will try to honour it but please give me some time.