It depends why it's doing it. So a list of all the work you have done to the gearbox that caused the problem would be good. And what parts you changed if you did open it up. If it's never been opened then it's more likely a damaged part.
If you have had the gearbox in bits and replaced the bearings/bushings then those would a good target to start with. You might not have seated them fully, or they might have flanges that are too thick for the gearbox shells. Bushings/bearings should sit flush to the shell when installed.
The distance the nozzle is retracted would indicate the bearings/bushings as the culprit, simply for the fact an 7mm bushing will push the tappet plate back 4-5mm if the flange is catching.
You need to rule out everything else as well so it's not as simple as 1 solution will fix it, you might have a badly shimmed gearbox, Damage to the tappet plate, Or something else might be catching the plate.
