Work on your shading. Have a determined light source. I prefer using the upper left. Idk why.
But ALWAYS have a light source and keep it there. There are many hats I've seen without a consistent light source, which keeps changing (example: 2013 fireworks hat)

I also have noticed you have the same problem with the ones at the top. Try drawing a sun at the top left of each square or something.

Example using your work: For the top left hat, when facing up the light is in the middle. For the facing left one, the light is coming from the right.