That last portrait is looking nice, values are looking better. Two things: Watch the overall height to width ratio of your entire head, in the last portrait your face is a bit too wide. It looks like you need to just take a bit off the right side (from viewers angle)
Try using bigger brushes when you do these digital paintings. Try picking the overall value/color of an area and paint it in with a big brush. Then come back over that with progressively smaller brushes to tweak it how you want it.