You have some good pictures here and they show great progress. Admittedly, you're not the best at doing hair but the facial features look great and your proportins are accurate. Good work.
I like these a lot. Hair can be tricky because it's hard to delineate; it's just something that takes some practice. if you can be sketchy with hair - rather than putting in definate, flat lines - then it really helps later on when you want to go back and make things concrete.
My one general suggestion: shade more. Even if you do so lightly, or do it with crosshatching or whatever, it really helps make portraits seem more realistic and look more like the person. The shading gives form to things like jaws and cheeks and chins and foreheads, which you can't really put in with line, but are very important to getting a picture of someone to look like that person. It might seem strange at first, since skin - even dark skin - is something that most people want to shade very lightly or not at all. You have the proportion and the features down - that stuff is excellent. And there is character here as well. But to get the most out of the character and the uniqueness of the face, more shading helps.
