The taper on the crankshaft or the taper inside the flywheel are damaged or not the exact same angle.  If it shears the flywheel key it usually messes up one or both of these tapered surfaces.  Remove the key and use valve grinding compound to lap these two tapered surfaces so that they are an exact match.  Thoroughly clean the valve grind compound off the shaft and inside the flywheel, install the key and torque the nut to about 50 to 55 ft lbs.