Push your values further. Right now it looks like you only have 3 colors; shadow, base, and highlight. I would go back and add another value of shadow and two more on the highlights side. This isn’t to say “make something darker and two more brighter spots”, but you need to increase the general range you’re using. Because darker colors are visually heavy, you don’t want to weigh a picture down trying to ‘balance’ the amount of shadows and highlights you use.