Many painters get stuck on contour and see only 2d shapes instead of drawing and painting the solidity of the forms they are trying to translate to canvas. When an artist sees everything as a flat shape they tend to ignore the visual clues that help give it more dimensions. You can teach yourself to see the form of a thing by doing what we call drawing through the form when sketching.
Drawing through the form uses construction lines to further enhance the 3d effect of an object and to set its depth clearly in your mind so you can then translate it to your canvas. It uses perspective, geometry and basic shapes to build your subject with. It is a constructive approach but can be used as an observational approach also.
It’s a good idea to practice sketching like this at all times until you automatically visualize everything you see this way. Some people prefer to start with red pencil or blue pencil to separate the construction lines from their refined final drawing. Doing this allows you to pick out what’s important to you.
Illustrators and production artists are aware of this approach because many times they are inventing scenes and characters from their imaginations but there is no reason for gallery painters not to use this tool also. Many problems can be avoided by paying attention to volume when drawing a landscape, figurative or still life picture.
(images from top to bottom Will Pogany, Peter Helck, Armand Cabrera all images are copyright to their respective owners)