Dave,
From what you wrote, I assume you tried changing just the 30 tooth. If you can make the 'bad' gearbox work with the 30:45 set out of the 'good' gearbox then it is the gears not the center (centre?
) distance.
It is possible to get a bad gear. I have run my gearboxes without load, no lube to break them in. Some of the 20 PA boxes have been tight. All the gearboxes I have built with 14-1/2 PA have not had problems.