You seem to neglect the backside of some of your sketches a bit, which ends up a lot of the time (as seen in other heads by other artists) in superficial details that
do look good, but don't bring anything new or substantial to that space that makes it great. I'd put just a little bit more thought and focus on what you could do to spice up the back side of heads (although you do already do this in a couple of the sketches - like woman robot). Just my personal opinion, so you can weigh it as much or as little as you like.
You
may have the faces a bit wide in a couple of them, but I didn't check mapping either so rip.
Otherwise, what you have down looks like a great foundation for those heads. Keep at it