Need advice on a parking brake repair...

Recently, my parking brakes stopped actuating - even when properly adjusted by the FSM. I think I found the problem, and I need some advice as to how to make a repair. It seems that my bottom brake shoe is slipping of the actuator when the brake is set. The metal "ear" on the shoe that rests on the actuator is slipping in between the actuator and the retainer. Once it has slipped off the actuator, it is then lodged between the retainer plate and the actuator and will not reset.

I am not sure how to correct the problem. The shoes do not seem to be worn. I replaced them about five years ago. The actuator does not appear to be worn. I am not sure about how loose the retainer plate is supposed to be.

Any ideas or similar problems?