I've got a new Rhino SPHD post hole digger with a 9" auger. The auger has a fish tail bit that is supposed to be good for hard soils with flat rocks. I'm using a New Holland TN75 tractor. I'm trying to dig in dry hard ground - fairly rocky sand stone formation. Even without hitting any rocks, the auger is barely digging into the ground. I'm running the PTO around 300 rpm and lowering the rear arms as far down as they will go but the auger just bounces around. I tried wetting the hole, but it didn't help. Anybody have any ideas??