Hi! I think that the main issue that the head might be a lack of volume. It seems like the features such as the eyes and mouth are sitting on a flat surface, while the face curves a bit so the features should show that. Also the eyes are spheres themselves, so the form of the eyelids and shading should reflect that more and it will help them not look as flat.
Also, would recommend being a bit bolder with your shadow by making them with harder edges. I feel like it's easier to map out the volumes this way and I also feel it's easier to go from hard edges to soft than the opposite.
I hope this helps a bit!