So the alternator will, or will not, slide in the notched bracket? If it's just loose and wiggles but won't slide in that notch, then there's a bolt (or nut) that has it locked down. I seem to recall scratching my head over this a while back but can't remember which Kubota tractor it was -- point being, it wasn't overly obvious. Seems like it was actually part of the tensioning bolt mechanism on mine that locked down. I'll have to go look now.
If the alternator is sliding in the notched bracket, then I have no more suggestions. But since you say it won't move closer to the engine, I assume it's not.