I have a 2014 +4 and I found the problem with a non-sliding passenger seat was down to a mismatch between hole centres. The distances between centres of the mounting bar below the seat runner was about 3-4mm than the distance between centres in the chassis. Slightly elongate the holes in the bar with a file, to match, and also countersink slightly the floorboard under the carpet to allow for the boltheads mounting the seat runners. Problem fixed.
