I bought these bits to replace the original ones that came with the auger which had wore out because of all the rock, and they fit like the original ones. Made the auger seem as if I was using a new one.

If your digger isn't any longer digging the cutting blades probably need to be replaced. Once the outer edges are worn off far enough instead of getting a purchase of dirt as the auger spins, the auger will just spin in place. It will be riding on the nut and bolt with which the cutting blade is secured actually holding the blade away from the soil, therefore, it can't get that first bite of dirt that continues to pull the auger downward. ...or you can just keep climbing over the back of the tractor and jump up and down on the swing arm of the post hole digger!