Hellu! I've seen you improved your head dude, but you can still improve a lot of stuff:
_Your hair doesn't really look realistic, you should look at a pretty cool tutorial on deviantart about pixel art hair.
_you can shade your skin a better way, I've seen you drew the shadow of your hair on your forehead but it's not enough, you can enlighten the side of your nose and your upper lip, add a darker shade on your cheeks, your lower lip and the inside of your ears. If you do this, the mouth will look way more realistic than a black line.
I hope I helped you, I'm showing the newest head I made in case you would want a model. It's not the best head ever but it is the best head I did make.