Value differences are created by adjusting marks, shapes or areas in your art to have greater or lesser contrast, or in other words to be either darker or lighter. This contrast between light and dark is relative. Shapes or marks will always appear darker next to something lighter.
As a general rule, there is a tendency for the values to go more and more towards middle values (midtones) as you paint. You almost can’t avoid it! Having an awareness of this will help you paint with more distinct light and dark values in your art. It is also helpful to paint a true patch of absolute white and black on the painting, even temporarily, as this will key you into using the full range of values and not just the middle values.