Cranman knows his stuff, as do most all of the posters here, and I think he's absolutely right about what might be ailing that little 521. But just to be sure, you can do a little test. Keep in mind that the impeller spins many times faster than the auger so you need to spin that impeller quite a few revolutions before you see much movement in the auger.
If I suspect a problem in the gear box, I attach a set of vicegrips to the auger in such a way that they touch the ground. Then as I turn the impeller I wait to feel a resistance back through the gear box due to the auger being stuck by the vice grips. It may take a few turns, but you should eventually be able to feel the resistance. Of course when you encounter the resistance, don't force it any further. Those brass gears aren't very rugged.
Little or no resistance?
Do as Cranman suggests. Make sure there's enough grease in the gear box before you button it up back together. Maybe you can even save the 15 bucks on the gasket, and make your own from a cereal box like Donyboy73 did in one of his videos......
Good luck with it!
If you're looking for guarantees in life, buy yourself a toaster.
--- Clint Eastwood