Seat belts in Chevy/GMC vans

The program for mentally-handicapped people where I work owns about 20 Chevy/GMC vans. The problem is with the locking retractors that get stuck and keep the belts from being unwinded from the retractors after the vans are stopped to pick up and drop off people. On both sides of the vans, the shoulder belts hang from the pillars near the roofs. On bench seats, the shoulder belts for middle passengers are attached to the retractors on top of the seat backs. How can we fix stuck retractors without having to buy new ones?

Is the belt hanging down and not retracting, or will not come out so that it can be buckled? If it is hanging but not retracting I can give you some fixit tips. I have had to rebuild three for our fleet vans.