Dreadlocks become flat for several reasons. The most common are very wide sections and pressure, such as laying on them.
To prevent or fix this issue you’ll want to start with sections that are as even as possible. Some examples are squares, or equilateral triangles instead of long rectangular sections. If you already have long sections you could consider re-sectioning or splitting thick dreadlocks by an experienced professional such as ourselves. Other solutions include palm rolling and using beads when the hair is damp to compress and force the dreads into a cylindrical shape or sometimes wrapping the length with string can help.