Rading this and I think I understand what is happening. The zoom ring at the back will only go to 20x mag at most.
I did have a very similar problem on my MK2 which was the elevation turret was interferring with the zoom ring mechanism when the turret was dialled into the 55+yd targets, could only get to 30 mag. I got around this by firstly recentering the elevation and windage turrets to the mid point. I then fitted a set of the ATP61 adjustable mounts.
I set a target up at 45yds, leaving the turrets in the cetre point, and used the mount adjustment on the rear one to get the pellets within 1/2" maximum of the aimpoint. Did have to get a set of risers as the front shade touched the barrel.
Once doing that I have not had a problem (well apart from me at times
) with the set up.