The blanks are not being squared to the tube before (or after) turning. It is important that it is SQUARE. Some skilled and experienced people can eyeball it, but most people need "something" to square the blank to the tube on the ends.
An example is this at PSI:
https://www.pennstateind.com/store/PKTRIM7C.html
Or this:
https://www.pennstateind.com/store/PKSQUARE.html
Rick Herrell, a member here makes a sanding mill, an older version here:
www.penturners.org/forum/f167/sale-economy-sanding-mills-42098/
And I think this is the newer version:
CORRECTION BELOW (Sorry, I got multi-clipboard mixed up on the original link.)
These are different tools to square the blank, otherwise, un-squared blanks will cause the problems you described.
A bit of a warning, while many people have the carbide cutters for end squaring, they have a tendency to "bite" on occasion and cause other problems for many. Sanding it square with sanding mills are much better on the blank and safer in the long run. Experienced people can use a cutter OK, but inexperience can lead to more problems.