You have a defective tumbler - contact the manufacturer, tell them the problem. You will probably have to send them the lock. They should replace the tumbler for you free of charge, unless they determine that you where somehow at fault. If it's a used or old lock, you may have to fit a new tumbler.
I have an antique Manton shotgun that broke at some point at the same place, and was repaired by brazing the tumbler. While it appears to be sound, I don't completely trust the repair.
Good luck!