What I think your doing wrong is your not keeping a steady brush and your highlights are uneven and blotchy.
Try starting off with a 2:1 mix of Codex Grey - Chaos Black, make sure to keep your highlights straight and full, then you can add a high light of pure codex grey onto the model but be sure to leave more of the first mix there.
I quickly took a picture of a model of mine in the bathroom (

) But it's not the best picture, hopefully you can see where I'm coming from
Hope this helps.