I think some people have said here on Bluelight that some of the kratom alkaloids have a longer half life and last longer in the body? So maybe the WD doesn't start til those alkaloids are mostly gone. I doesn't seem like those particular alkaloids deliver much in the way of a noticeable high, since the enjoyable part of the high wears off so quickly, but maybe some sort of effect is still going on. But I really don't know.
How long and how fast are you weaning down?