When we first picked up my dads n16 (almera) we had an issue with the passengers seat belt not retracting properly, it was twisted and was getting stuck as it tried to roll back up.
Same thing happened to my rear passenger belt when I un-bolted it from the floor (to change the rear quarter trim) and I'd bolted it back with a twist in it *facepalm*
Or as Andy (h701micra) said it could be an issue with the unit that is supposed to retract it...?