The belts have an inertia lock on them and it seems your lock is getting stuck in the lock position when on rewind. Yes, a slope or incline will usually unlock them but it is suggesting that the belts need attention or replacement. Initial idea is to lock the belt into the buckle when you get out of the car. Next step is to fully extended the belt when you can and slowly retract it, do not let it wind back quickly. It's a good bet the reactor is out of calibration, for whatever reason, and may need to be dealt with at a dealer.