I don't think it's the soil, since he said he could out-dig the tractor with a shovel. My neighbor and I jointly own an ancient ratty PHD and routinely dig 3 or 4' deep holes in clay filled with boulders. There's no downforce, just the weight of the PHD. I think it's got an 8" auger on it. What I would do is borrow or rent a different PHD and try it on your tractor. Assuming it works, compare the two devices carefully to see what is different. HTH.