Usually, the problem is at the very front of the arbor. If you look at it close, there are file marks (little flats) from a half hearted attempt to fit it at the factory.
Clamp the frame in a padded vice or with a padded clamp to the edge of a work table. Wrap a slice of 220 wet or dry sand paper around half the arbor and play "shoe shine." Flip it over and do the other side. Test fit often.
Coffinmaker