Can't mount Canon FD lens to FD adapter

I have a Canon FD 300mm F/2.8 Fluorite S.S.C. and I want to use it on my son's mirrorless camera. I got an FD adapter and when I went to mount the lens the metal collar on the lens would not rotate and so the lens could not be mounted to the camera. I tried a couple of other Canon FD lenses, including my other Fluorite, and there was no problem. This lens works normally on my old F-1n . However, on all my other Canon FD lenses the metal collar that is used to tighten the lens always automatically rotates slightly as soon as the lens mount engages the camera mount on the F-1n. On this lens the ring does not rotate automatically at all. You have to be careful that that the lens is aligned properly before tightening things up. So my question is what do you think is wrong with my lens and is it an easy fix?

Are you sure that the locking coller is turned all the way clockwise as see from the rear and that it locks in place? Edited August 24, 2021 by William Michael

Are you sure that the locking coller is turned all the way clockwise as see from the rear and that it locks in place?

Edited August 24, 2021 by William Michael

Yes, I use the same technique to mount this lens as I do the other FD

lenses.