Iirc the fronts are pretty much fool proof when fitting because of were they sit but rears can be moved.Its 5 years since I changed all the belts in my Sapp but I had an issue with the rears.
Slacken off the bolt that holds the seat bealt assy to the car and run the belt in and out see if that does anything.It may of been the top mount I had a problem with I think as there is a plastic washer there and it was binding a little bit and not returning to where it should of been making the belt return slowly.

Does this make sense to you lol.