could be a sheared key in the bronze tiller shaft gear, or could be the teeth on the gear are broken. May even have to replace the shaft that runs the gear. Before removing the tine shaft check to see if there is wiggle in the shaft. If so then you will also need to replace the tapered roller bearings while you at it. if the seals have cut in the shaft then will also need to replace the shaft due to that is where the rubber part of the seal runs. have replaced several over the last several years in the shop.