FWIW I've just edited two shoots which were both shot with two XplorPros, the R2Pro trigger, a Nikon D810. I combed the files to see what proportion of images were dark due to misfires. Here's the numbers:
Shoot 1 - personal work in studio:
Look 1 - 0 for 349
Look 2 - 1 for 584
Look 3 - 6 for 598 frames
Total 7 of 1531: .0045% // 99.9955% - that's a pretty good number I think the uptick toward the end was just prior to having to swap in a fresh battery.
Next shoot was a celeb portrait session:
Look 1 - 0 for 389
Look 2 - 2 for 223
Look 3 - 0 for 101
Look 4 - 0 for 119
By look 4 I beat the recycle on the fill light a few times, for a slightly underpowered fill, but it's nothing a little massaging in post can't fix.
I think these numbers feel average for me in my experiences working with the Pros, so it's strange to me you're seeing such terrible consistency. I work usually with the lights around 1/4 to 1/8 power, and often even lower. Not sure if you're at full or half, but... that could be a contributing factor (you may have already said but I'd have to reread the thread to see...)
Please keep us updated with your efforts and hopefully solution.