A toilet flange is supposed to set on top of the finished floor. The wax ring (or the waxless ring) if fitted to the horn on the bottom of the fixture so that when the horn is inserted into the flange, the sealing ring forms a tight seal between the toilet and drain. Two tee bolts in the flange secure the toilet to the flange. Sometimes a floor is not perfectly level, so it must be shimmed to prevent the toilet from moving even though it is bolted down. That's all there is to it in a perfect world. Unfortunately, we don't always have the perfect world. If the current floor was laid after the flange was secured to the drain, it could be sitting lower than the surface of the floor which could require 2 wax rings. The fact that the shim is no longer working lead me to believe you should pull the toilet and see just where the flange is resting. After cleaning the old ring off, try setting the toilet on the flange without a ring and see what shimming is necessary. Then install the toilet with a ring or rings and apply the shims as your test indicated.