Help me improve drawing the human face please!

I been trying to improve on drawing the human face for a while now but I seem to be stuck when it comes to the improvement department. It's like I'm stuck and can't improve, I need some tips and tricks to improve my drawings. I draw from reference most of the times such as Google images and such. I'm having trouble getting the exact likeness of people in my portraits.

Here's the drawings: http://imgur.com/Lu1Q1VM http://imgur.com/uW1M8p3 http://imgur.com/YguVDyc