Dude, that's exactly what makes your work so awesome. Don't strive for portraits with everything perfect. I love your style with the messy strokes forming exactly right to recognizable portraits. Keep up your personal touch! Cheers
This actually looks great. You say you need more practice, but it all the depends on the style you're going for. A little more "polish" wouldn't hurt with these, but all in all they're pretty much well done.
But what might notice (not at first, mind you) - the left one lack detail,might be a bit sharper. The middle one looks like the face (or hair dunno) was glued on, like the shoulder/face/hair were on different paper pieces and were cut and pasted together. The right one looks best IMO, can't really say much about it.
I am nothing close to an artist, so my opinion is worth what it's worth, but I think a viewer opinion might be of some help... I hope you don't mind :/ I feel what you mean when you say you need practice on your portraits. Your usual drawing style doens't really fit what one awaits from a portrait. They're beautiful (sincerely, they are!), in a strange kind of way you don't expect in this kind of picture, intrinsic to your particular design.
Exploring yourself to find how YOU want to draw portraits would lead to an improvment for sure! Plus maybe a bit of practice on their expressions, which are already nice but could be perfected, in my opinion.
Now once again, I couldn't draw if my life depended on it, so you're free to spit at my face and opinion, I guess I'd have more or less deserved it by judging something I can't really judge x)
Anyway, I love your work! Have a great day man, and keep the awesomeness up!