The passenger belts have a ratchet that engages after the belt is pulled out more than a certain distance. The ratchet won't disengage until the belt is nearly fully retracted. It's supposed to be for child safety seats, to keep them firmly in place.
My wife runs into this all the time, as she's one of those who pulls the belt out all the way before latching it. If she pulls the belt out just enough to latch it, she has no problem.