Agreed with marcgfx, you have a bit of a problem with soft contrast in almost all of your examples. The shapes that you have defined with shading themselves are really good, and I can see you have a really strong grasp of dimension. But don't be afraid to have a greater separation between dark and light. The dragon especially, which I would assume has scaly, and hence shinier skin... if you made the shadows very dark, and the lightest part of specular light almost white, then it would give the piece a much needed sense of depth and texture.