some nice stuff is going on here, mate!
I'd say the main issue here is the lack of solid knowledge of skeletal anatomy that leads to structural problems with almost all your portraits. You really should draw a bunch of skulls, man - or take Loomis' heads and hands and learn how to construct a face - instead of hardcopying them one feature at a time. This last one is a good example of what I'm talking about. The left eye is totally out of perspective. We shouldn't see its inner corner from this point of view. The left brow also suffers from perspective problems. We shouldn't see its outer downward curve of it. Generally speakin': If you want to be successful with realistic head drawing laying down the structure must be the first step.
Another general problem is the overdone smooth gradients. Like in this case.Apart from the fact that the left side of the face is out of perspective again you toned the face in a way that it looks almost like plastik. I'd say you should do some flat, cell-shaded renderings with no gradients at all. You can learn a lot from it.
Long story short: Doing studies is a good thing but learning from the masters (and not just copying photos) is even better. You should really try that way too. This can be a good place to start.