Paul,
I spent two hours on the phone with powermatic on the same problem. They were very nice and we checked all the things mentioned already. All was in alinement, I use the PSI pen saver mandrel. Works very well have done many pens on it. I am looking for bushing to go to TBC.
They carefully told me that it was me and not the machine. They were kind. The issue was my roughing gouge, which is what I use was a little dull. They had me sharpen it up and had me make very light cut by turning my wrist to engage the acrylic. After a few minutes the blank became round. Basically I had pushed it out of round and it will stay that way as long as I keep my tool against the pen blank. It was not until, as they said, I cut so lightly I made a fine cut that made flower, not a ribbon of acrylic. As I said it became round again. I guess I was to much in a hurry and was pushing hard to get the cut. Since then all has been well, good round pens. I do lots of euros and had the same issue with the tenon. It was some times out of round, because my parting tool was not as sharp as it should have been. I would rather turn than sharpen. I hope that helps.
Tom