Experiment with values, not all "value" is blurred solid black or white. They also don't follow a real light source. You throw "shading" on every line. Your heads look flat, and they all look the same. You've used the same mouth from that google search video "how to make toribash textures" yet again.
The teeth on the eighth head look bad because they overlap but the color tells you otherwise. Spend more time on one head instead of calling it quits so early. You don't seem to have a ton of creative ideas, so make what you do come up with look good.