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I see some fun experiments with chin shape in this drawing! That will give your faces lots of character. :) But it looks like most of your head shapes are missing the top part of the skull; remember that head structure does not stop at the hair line :)

I see you sketched out some faces here. My advice is to think of the head like a 3d object and not just a pancake. It should have mass and not just hang around like a peace of paper. (this is a friendly advice, don't get me wrong) I recommend the Loomis method, it was really useful for me.