Your centerlines in some of your heads should be curved more at the top remember that the base starts as a sphere so the centerline should be like a crosscountor in a sense at the top but it connects to the center of the chin and goes off the circle as it comes further down. The ear seems a little small to me the neck should have some bend to it it isn't a rigid cylinder. The head in the bottom right wouldn't work and looks weird because the neck isn't bending at all. Your side circle should be more on the side as well it looks like it's intruding on the top of the head. Great practice over all keep up the good work!
