These parts are far from flat/straight and the instructions say to flute, but how in the world are you supposed to flute material that doesn't exist due to the scallops? Am I supposed to flute closer to the holes? Or do I need to grind down the tool so it reaches further in on the flange?
Probably doesn't show up that well in the photo I guess, but that was the problem - there was exactly zero engagement of the upper tang at that point. I would have had to grind down the lower two tangs of the fluting tool to get even the smallest amount of bite.
I have a fluting tool with nylon jaws that easily grips the full width of the narrow flange. Maybe ask around and see if you can borrow one with better jaws.