Hello,
Welcome to Vacuumland. I recently took my Model G to an Aerus dealer. They tightened the spring, and it works just fine now. A cord winder spring is similar to a window shade spring in that it can be loosened with use. And, like a window shade spring, a cord winder spring can wear out; but try having it tightened first. I've been told, though, that it is a risky endeavor; that you can get seriously hurt by a spring if you accidentally release all the tension at once. So you're better off taking it either to an Aerus dealer or a vacuum store if you haven't been trained.
Another tip for helping the cord winder work at top efficiency is to coat the cord with talcum or baby powder.