The antenna hole in the fender has notches that keeps the whole assembly from turning in the fender.
Sounds like the raised part on the antenna that fits the notch may be rounded off. Your gonna need to get a crescent wrench to hold the piece on the back side of fender while loosening the antenna using the flats. The top piece with the flats is a nut threaded on the the mast which fastens the antenna to the fender.
