Not a machinst by trade by mount some barstock using the tailstock (thick enough so you do not need a steady rest) and take a cut from tail to head, at least 6 -12 inches.
Measure the diameter at several points. You are looking to see if you are tapering the bar, if so then *something* is out of line. Your tailstock could be off center, a dirty ???.
If you adjust something just remount the bar, take another cut and check for taper.