I just recently developed an issue trying to trigger a YONGNUO YN568EX II Flash using wireless YONGNUO YN-622C-TX triggering YN622C II receiver/triggers.
So, the mystery is everything worked yesterday. I set up everything today and fired some test shots, and the YN568II flash would not fire. I have 100% wireless control over the flash (change from ETTL to M, change flash power, zoom). ALL controls work wireless as they should. Flash pops when hitting the pilot button on the flash, flash pops when ON camera taking pics, flash pops when hitting test button on the transceiver/controller, flash pops when hitting test on the wireless receiver. Flash pops ALL times, EXCEPT when I press the shutter. Not even sure how that could be, but it's happening. It's like all the complicated stuff works, but the easy action of triggering the flash doesn't. Also, yes, batteries are good.
Also adding that I checked the wireless triggers on my Canon 430EX II flashes and they work perfectly fine (so the triggers work). Also can fire the Yongnuo using an Off-Camera Shoe Cord. Everything works except using the wireless triggers to trigger the flash.