Untitled

The drawing looks clean, but the faces look a bit "normalized", rather repeating a standard, than actually working from the reference. One part of that standard seems to be quite big eyes, so I assume there is some manga influence. If you want to give your eyes a more natural look and size, I would point you towards really starting your eyes by finding the eye sockets in the skull and sketching them in, before drawing the eye itself. Maybe actually drawing a few skulls from reference might be a good idea, so you know which bony parts determine the planes of the face.

I feel as though the forehead on the upper right one is a it far back. if it weren't slanting so much so quickly it might look a bit better :)