More likely to be the joker than the flapper, but...
Recently a friend used the Raritan II head and it would not evacuate the water. Before I tear it apart I am wondering if anyone knows whether it is likely to be the flapper valve or the joker.
There are other possibilities too...I need more information before I give advice:
How old is the toilet? What exactly are the symptoms--increasingly hard to pump? Pumps maybe too easily, stuff just swirls in the bowl? When, if ever, was the joker replaced? Have you made sure that the tank vent isn't blocked? A blocked tank vent would pressurize the system, creating backpressure that would prevent the toilet from flushing.