George,
This may be a dumb question, but do you have the stepped/slotted bolt on the back of the rider's seat or just a plain bolt?
A bolt like this???
slotted bolt If so, and you're still having the problem, you could try a rubber washer or 2 under the rear of the pillion bracket where it bolts to the fender. If it "seeks it's own level" over time. you can just remove the washers as needed.
