Do you have the piece of 'saddle' leather fitted on that seatbelt bracket, attached to the top of the seat? I think it's to stop the belt chafing the seat leather. My Plus Six didn't have it originally, so I bought them from the dealer. Once fitted, I noticed it raises the belt by about an inch and the belt doesn't start dropping downwards until it's clear of the seat face. Might make a difference for you.
