I had same problem when I changed air shaft in my Reba. Mine was due to the damper shaft topping out before the airshaft which has the rubber stops on.
Not all rebas can go 120mm as I found on Web some have a shorter damper.
I got the guys at work to chop a bit off the air shaft so it topped out before the damper.
If you drop the lowers off and pull down both shafts the air should be shorter than the damper
Nightmare typing on phone hope you can understand what I’m saying