Originally Posted by AuSeeker

Unfortunately it could be the teeth on the flywheel, if they are rough/chewed up to some degree they may not be letting the gear on the starter slide back away from the flywheel, the way the starter gear is released is from when the engine starts running the flywheel is spinning faster which spins the starter gear faster than the starter motor and the starter gear is forced by the counter rotation back into the starter, if the flywheel teeth are roughed up the gear can hang up and not be forced out of mesh by the counter rotation.

The next time you have the starter off try looking at the flywheel teeth, you may have to rotate the engine 1 rotation by hand as you inspect the teeth because it may only be a few teeth that could be causing this issue, if you find this to possibly be the issue you either can try filing the bad teeth a bit to smooth them out in hopes it will let the gear release better, or replace the flywheel which will be very expensive job I'm afraid.