It sounds like your bendix drive is not engaging correctly and therefore spins and doesn't advance forward to interlock with the gear on the flywheel. My suggestion would be to take the starter off the tractor and clean it up real good. Check the bendix drive as well as the bushings to make sure everything turns and moves freely. To remove the starter you will have to take off the plate from the bottom of the tractor. Then remove the flywheel cover and undo all the linkages that run from the starter itself to the foot pedal engage. There are three bolts that hold in the starter that will need to come out to remove it from under the tractor. Be careful as the starter is not light be any means. Let us know what you find!
From Denton, Nebraska. Andrew Kean