I love this piece and it works as is but I do agree that it could use some more depth. One ay to do that is by exaggerating the shadows. Here Ishow what adding a multiply layer can do. You could play with the amounts, or you could just make your shadow color a bit cooler and darker. Either way, adding contrast between the light and dark values will add the depth you are after. It also gives you so dark to pull out even ore that glowing eye effect. I think if you play around with seeping even further your shadows you will get the depth you are after. Hope this helps.

