You are on the right track. A few things that pop into mind... Your lights are mushy and airbrushy. There can be more attention paid to the shapes the lights make. Avoid using the same brush all over if you want to improve that. And two...your lights are almost always white. Working one half value step darker in the lights and one half value step lighter in the darks will keep colors in both your dark darks and your light lights. Your color can also change as it moves through space. Right now your lights and shadows are often the same no matter where they are in space. Color movement is important.
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