It mostly boils down to the structure and proportions of the heads your drawing some of these look fine but other times the face is too long or too wide or thin. I would say try to first do the facial features by themselves(mainly noses and eyes) and use references to understand how they are structured, I would also revisit the head formula Marc teaches to help better understand how to structure and accurately proportion out the facial features and head at different angles. Most of your heads look ok, but a lot of them have minor or major issues in structure.
