I had that problem with a machine one time and had to trim it out on each of my headers to allow the latch to enter the heads. I thought either the heads or the combine must have been made improperly.
Had to do this too. The R-40 and the flex head worked, but the machine wouldn't latch onto the hugger. Fix: took a grinder to the plating on the hugger that matched up to the combine's. Sounds like you did something similar. In my case, it sounds like the hugger was made improperly. Have a good one.