does the ring move easily on the unit.
Have you put a screw driver in one of the lugs to help turn it while you have it there?
Cleaning off some rust would not hurt. Maybe a little Vaseline to make it slide might help.
Putting a screwdriver on one of the lugs and tapping it with a hammer can help drive the ring around the mount. Even new they can need to be snugged up with this method as you may not be able to hand tighten them. At least that is my experience.
Is this the same disposal that was clogged with molasses? It may be time to put that thing out of its misery.