The rear seatbelts do not clip into the slot. Im not sure if theres something inside the slot but you can only push the belt clasp so far in but it doesnt click. Can they be opened up easily to look inside?
First thing to do is clean them carefully and inspect for any foreign objects in them.
Then it sounds silly but are you trying to plug the right buckle into the right stalk?
If the vehicle has pre tensioner seat belt stalks and has been in volved in an accident the pre tensioner may have gone off and the stalk may need replacing.
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