The outer shaft had little scars in it where the screws have rubbed on it.
Those little scars mean that the lower head was slipping on the outer shaft, if they go around the shaft.
Then that's your problem.
The upper screws of the lower head need to go into the holes on the outer shaft.
Far enough in to keep the head from slipping but not too far or they will rub on the inner shaft as I mentioned above.
The lower screws just butt up against the shaft.