THe pink and gray shadows on the top and back of the skull are darker than the rest of her head, making it look like her head is "dented." If you can even out the coloring and depth values, it will help. Notice how the light source on her face is from the upper, near center right. Her bald skull needs to likewise show that type of lighting to be consistent.

The highlight above her eye should extend upwards to the front of her head. Try to refine the edges of her head, too...clean them up & give them a slight blur.