I personally don't paint
NMM, but i like to draw & paint 3D Objects on 2D surfaces.
I think what may or may not throw you off is that your shadows have no clear direction and your Contrast is to low. I know that
NMM - which you can also paint/draw on paper - has almost no mid-tones, and consists entirely of either light or shadow, with very harsh transitions between them.
Metal is also very reflective, so the lower metal orb would reflect the upper metal sphere and vise versa.
Shadows on a spherical object are a bit counter intuitive - You have a core shadow, a reflected cast shadow, a cast shadow, a reflected light and very little mid tones - here's an example for better understanding.