the shading throws it off
Certainly better than much of the other "fine art" on NG but the value scale you used doesn't seem consistent. Some parts are lighter than seems feasible and other parts are dark even when they appear to be on the top plain all of which makes it look less thre dimensional. Also the contours outlining the skull could be sharpened with more contrast so it doesn't look smudged, though it looks like charcoal so there isn't much hope of keeping it all clean then.
