If the 6 corners of the shaft get slightly worn or compressed down, then they will turn a bit too far in the hex 'socket' of the pump gearshaft before 'catching', and jam in. If it turns harder, then the shaft hex spins in the pump's hex socket.
Used one of the new ones. A test fit of the shaft was too snug so I just gently drove an old shaft in and out of the bushing several times using the old shaft as the burnishing tool. Loosened up nicely after 4-5-6 passes and spun freely. Put some moly cam paste and oil in there on the new shaft and moved on.