If a head does not work no matter where you put it it could be the led card or a contact issue. Sometimes debris or slight warping will cause it not to light up.
That version uses a metal spacer that goes over the screw inside the lens, it is the electrical connection between the screw and led card. You can try swapping just the LED cards between the lightheads as another test. Just make sure that all the components are in place when reinstalling (plastic insulator, spacer, correct led card orientation and gasket.
I have some red or amber cards available (but no blue) if you get to a point where you feel you need them. Happy to help ya troubleshoot it as much as I can too.