Hi Redtails,
A couple of students of mine have had the same problem. The 'easy to fix' cause is if the insert hasn't been screwed fully home so that the nose of the insert isn't against the machined flat, inside the chuck. The threads don't form the 'register', it is the faces of the chuck and insert that do that. Also check that there is no dust/dirt on either of those faces. Similarly, make sure the face of the insert is tight against the headstock spindle, when the chuck is fitted to your lathe. I assume you are checking runout of the chuck when it is on your lathe.
The more difficult 'fix' occurred (about six years ago) when some inserts were poorly machined and were extremely tight to screw into the chuck. The insert should screw in with only light pressure with the spanner. We only had this once so it is a rare situation but, if this is your problem, take the insert back to your supplier, as you've got 'old stock', and they will sort it out for you.
I believe there were also some insert copies being made and sold via a well known auction site. In that case I am not sure how you could redress the problem.
Hope that helps,
