The head look flat because there isn't much shading. And I don't mean simple draw-in-some-darker-lines-at-the-bottom shading. It needs some thought out shading. Realistic shading.
Also needs material at the top and bottom. The Top at the very least.
It is, to me, the second most important part of a head, coming in behind the face.
While the face is what usually defines the head, the top is what you will see the most.
Robots also have few shaky lines.
The suggestions on line tool/pen tool would help a lot.
if you can't be bothered, then AT LEAST construct the lines from a series of straight lines. In GIMP, it's made by holding down the shift key while clicking. I'm not sure if this works in PS, but it has worked well for be in the past.
Also, to make it as realistic as possible, you should use the black lines as a guide.
If you observe objects in real life, none of them are outlined, like they are in cel-shaded tb :]
Of course, if you are going for a cartoony look, those work well
it's not too bad of a head, but it can be far better