Coupler nut size

I just tried to assemble my z-nut trap and coupler nut and realized that the nut is way to small to fit well in the trap (see attached). The coupler nut works fine with the normal nuts and threaded rod, is there a huge difference between the outside dimensions? It doesn’t look like there is any way this can fix even with a bunch of tightening.

There are 2 versions of that part, for 11mm and 13mm flat to flat coupler nuts.