A way to determine where to put (zenithal) highlights is by holding your model under a single strong lamp and look where the model is the brightest and where the shadows are the darkest. This works best on a bare mini where you can see the light bounce of the metal mini. You can still use this technique on a painted mini though, but sometimes it's a bit harder to see where the light bounces of the surface.
You can use this 'technique' as a guideline or when you are in doubt of where to place your highlights.
Your grey parts look pretty good as a intermediate colour, but they look so blocked in. The darkest areas are too large, and too big in contrast with your medium grey. If you leave these areas small, you create a transition for the brain/eyes.
Anyway to improve, I would go a lot further on the highlights for starters, toward the higher armourplates. Depending on what you want, you could go for deeper shading too. Just use thin layers and work your way up/down.
When you're done with highlighting and shading, you can glaze the area for smoothness. I don't see this as fixing anything but as a usefull painting technique to get a smooth endresult. Be carefull to use very thin glazes or else you'll lose the contrast you build up with highlighting/shading. The colour is personal choice. I often use a colour when I glaze a grey. Something like a darker blue or green. It gives some colour variation to a monotone area.
Just don't be afraid to experiment with colours and techniques!
