This document teaches about all the possible known causes for filament thickness deviation. Please note that there are so many variables related to this problem, that a 100% fix is of course unrealistic. But by reading this document you will at least learn what these variables are, which is the first step in fixing the issue.
A very common problem during filament extrusion is that the filament produced with your filament maker is oval or flat shaped. Filament ovalness (or flatness) can cause 3D-printing problems. This deformation of the filament can have two different causes, which will both be discussed here, as well as potential fixes.