The biggest thing I noticed in all your work is the lack of confidence you have in your line work. Your using a lot of "chicken scratches" to create a shape. Really try and train your self to use very few stroke for a shape. Looking at the last piece, you should aim to do the different clumps of hair in 3 brush stokes or less. One stroke for the bottom part and another for the top. Hope that makes sense. So yeah, my advice would just be to work on your line economy.
