I think that you are over-shading some of your drawings unnecessary,
in this case you overshaded the torsos.
A few hundred more from these
heads like you posted above, and you will be a master head and portrait drawer.
I don't know, currently you are still struggling with the skull shape, sometimes
the faces tend to be flat or deformed on some way, but you definitely improved
with the eye positioning. Hair, still do not fit all the time on the head, and try
to solve somehow differently the hair rendering, currently the girls look like
they have a messed up hair or like they are going half bald. Leaving white
blank strands between the other hair, creates the effect of baldness.
Look up for some artist who render realistically hair with graphite pencils.
Actually it does not take up too much time to make it look real, you just need
to know the right technique.
The last female head, the stand alone one, have some major issues,
I do not know what, but it looks real strange, maybe too long face, or
too short skull, the mouth is more protruded than it should be, I am not sure,
but it creates an awkward effect.
Improvement for sure, and you should be satisfied with your progress,
you just need to push things more forward just as you did till now.
Wish you a happy drawing, cheers.