well, first thing i thought was that shading of hair and face don't fit in terms of lightsource (hair: topdown, face: from somewhere on the left)

the shading of the hair itself is pretty cheap, you should not only fill the lineart with something straight 'shaded', rather than that you should use the shading to give some shape to the hairs / separate the hairs (or better straids of hair) from each other..

the face: deffo lacks of contrast. i know, that, especially if ppl are new to that field, they're quite sheepish in adding considerably darker colors, but thats the only way to create volume, trust me =).. it also proved well in the past to not dither faces that much

Thanks a lot for the tips. I just started experimenting dithering, so I don't really know what I'm doing most of the time, but I'll try doing what you said. Thanks again