Way back in message #@, RJ suggested throwing some gravel into the hole. I wonder if anyone ever tried that trick and how it worked? It sounded good to me, because it seems like some hard sharp fractured gravel might help to tear up the clay. We don't have clay here - except for a few cherished little pockets of mostly sticky blue clay. So I don't know much except to use the right tip angle, lots of pressure, and very, very slow rotation.
If the gravel trick doesn't work, I'd agree with others to take a close look at the post hole drill bit for the angle that the tip and the cutting edge makes with the hard clay. Like any drillbit, if sharp but held at the wrong angle all they can do is skate.
When carbide won't cut clay even with lots of pressure and dead slow rotation, then what else could it be except the cutting angle?
rScotty