The pussification of America
You guys wear seatbelts? What a bunch of women!
I've seen two (actually, now with that Armour-All gag, three) distinct ways posted to solve this. One was long and tedious and I don't remember it, but the other, which may or may not be the SUTHNR method, was to use a razor blade to scrape off all the crap that builds up on the plastic shoulder hook where the belt makes contact and curls around it. The poster said that something on the seatbelt gets deposited on that hook every time the belt runs by it, and eventually there is enough build-up of whatever to clog the belt and stop it retracting.
This question comes up often enough that, if somebody cares to come up with a useful solution based on real life, let me know and I'll FAQ it.