/ic/ - Artwork/Critique

Can any anons comment on the extent to which the right two heads have perspective or "three dimensionality"? The leftmost head is one I posted last week for reference, and at that time anons told me that the drawing wasn't convincing as a 3D form, and I've received that feedback on some of my other posts too.

Since then I've been trying to convey more realistic forms by paying closer attention to the placement of features, as well as by applying some basic shading to try and show depth. However, I think part of my problem is that I struggle to identify what exactly it is about a drawing that makes it look 3D, so I haven't been able to self-critique as well as I would like.

Am I on the right track with the progression of heads here? Any feedback would be appreciated.