Seat belts work on a fine toothed ratchet system. When you pull a little too quick they lock and drive you mad. This can happen more with wear and is more prevalent in older cars.
Modern seat belt systems have explosive tensioners which actually in a crash go off and tension the belt to slow your rate of deceleration. They are designed to work in conjunction with airbags.
Don't ever try to repair one unless you want tiny pieces and springs flying through the air. It's just easy to buy another one, new or used and they are relatively easy to replace.
If you are going to replace one and want any advice or help, just drop me a mail with your year and model of vehicle and I'll send you the steps on how to do.
This could be one of the reasons why I ride motorcycles as much as I can, that is until some smart ass designs a seat belt for bikes, ha,