Don't know for sure on your model, but most shoulder belts the lock is performed by a combination of rapid extension and a pendulum mechanism . there can be a contaminant in the works so if you have the unit out blow through the unit with compressed air, with the belt fully extended, use silicone spray on the belt it's self, and when re-mounting make sure the retractor is square to the floor, if that's the way it mounts.
If the retractor is designed to be mounted straight up and the carpet is tilting the unit then the pendulum lock gets touchy.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.