Adding the clinical framing, because it changes how the question reads. There is a difference between no evidence and evidence of no effect, and this subject is one where the two get swapped freely in both directions.

wendy_avl said:

If you are going to change something, change one thing and give it long enough to express itself.

There is a second half to this that has not been said yet. If two explanations both fit, the useful question is which one predicts something the other does not. That is answerable; arguing about which sounds more plausible is not.

Adding the numbers, since they settle part of this. Steady state is the thing most people miss. The terminal half-life is about a week, so every dose step takes four to five weeks to fully express itself. Judging a step at day ten is judging the ascent, not the plateau, and it is the single commonest reason people escalate before they needed to.

A narrower follow-up, since the general answer is now clear:

How much of the between-person variation is pharmacokinetic and how much is just adherence measured badly?

Moderator note: two posts asking for a source have been merged into one. Please search the thread before asking again.