I've had the same problem. Non-concentricity. In my case, at least one of the causes, I think, is a very slightly bent pin on my PSI TBC mandrel set. The pin on the live center (in the tailstock) is ever so slightly angled. I think that means the bushing is oscillating (shifting) while it rotates, meaning when I turn the blank down, one side is lower, the other higher, according to the shifting of the bushing.
I've purchased a simple 60 degree cone dead center, to use with my 60 degree live center, to attempt turning between centers without any shaft at all. The cone shape of the centers should allow the bushings to slide up until they are snug. I am not certain this will eliminate non-concentricity issues on its own...I may have to make additional adjustments (i.e. figure out whether pulling the tailstock towards me, pushing it away, or maybe twisting it, when I lock it down, will in fact give me the most concentric turning possible.
This was in fact the topic of my first thread here, and the very reason I am here! Out of round issues, which in fact I think, based on the responses to my thread, are really non-concentricity issues most of the time.