The need to represent the third dimension in paintings is sometimes overlooked, particularly by beginners to the medium.
Working in two dimensions (paint on paper or board), it is easy to forget that we need to create form in three. Omitting brushstrokes that travel around the branch of a tree, adding shading to each extension of the human body or natural phenomenon results in flat surfaces. Shape is created by shading, light to dark or vice versa. Abstract forms, are the exception to the rule. No shading is required.
Beginners can find the concept of the third dimension difficult and, thus, wonder what is missing from their work. Form and depth.