I am interested as in why it isn't getting darker. One theory I would say is that it comes from a long line of hmm how can I put it "pure " captive bred ball pythons and the effect of darkening has been reduced and reversed.

I personally won't be worried becuase some breeders try to purify their morphs every year and one of the goals is to reduce darkening (at least for me it would ).

You should be considering yourself to be lucky