I agree with drive_not_55. The tabs on the lock ring that engage the tank aren't flat, there are indentations that lock the ring in place when it's seated.
I had to hit my lock ring harder than I wanted to, but it only to one or two strong strikes to unlock the ring, then some lighter strikes to fully remove the lock ring.
I actually had more trouble getting the new lock ring seated after installing GT500 fuel pumps.
2005 Performance White Mustang GT Convertible, Steeda Sport Springs w/Koni Sport Shocks, C&L Intake w/Bama 93 octane tune