seatbelt question?? (can no1 help??)

[az_mk]

Hi got a prob with my pass front seatbelt, wasnt working at all would not spring back and tighten, so went to breakers and bought a fully working replacement, same side off a lupo (exactly the same belt and pretentioner) fitted and works fine sometimes but then other times you cant pull it out because it just locks itself, very annoying, there is no play from what i can see from fitment, doing my boggle in!!!!! any help would be great!
muchas gracias in advance!