I ran into that problem if I tried to put a nut on the "wrong" pushbutton. I'm not positive about this, but since I had old pushbuttons (like 9 years old) and new ones (all from happ though), I figured there was a slight difference in the 2 production runs for whatever reason. For the nuts that were hard to get threaded on there, I just found pushbuttons that they would fit on. The other nuts that were ever so slightly larger fit on any of my pushbuttons, but the smaller ones only fit on a subset of pushbuttons.
Try that - I bet that will solve your problem!