A drawing is successful if there is a sense of LIGHT in it.
Ironically, you create a sense of light by developing the shadows.
Every shadow has variations of darkness.
The photograph is the standard of reality in the present age - by mimicing the eye.
The EYE sees the world in a pattern of light and dark, and so does the camera.
The camera captures the spaces between things that bring out the reality of the objects in the image. The eye recognizes shadow shapes.
Because you can think (unlike the camera) you have a brain that separates the world into things, and so you miss what the camera records, and what your eye actually sees.
There is a hierarchy of lights and darks, a vast range of tonality, and ambient light too.
Look for lightest light and darkest dark: everything else is between those in value; everything else is moving toward light or dark.
The only perfectly white areas are highlights: everything else is darker than pure white.
If they eye and camera see translucency and transparency, then that is available to you in your art too.
The edges of shadows: Where are the sharp edges of shadows? Where are they fuzzy?
Shadows on rounded forms are darkest nearest the light, and then are grade off to be lighter with the bounce light effecting them.
There shouldn't be outlines. Sometimes you do not even realize they are there.
Push the value range.