jkmnop

...also, I have multiple YN-560IV flash units and the behavior is the same with all of them. The all work perfectly with my X-T2 and Canon 6D bodies and all fail as you all have described with the Sony.

Odd. I'm having exactly the problem you're describing with the YN560-TX on my Sony a7rii. It flashes once and then some sort of adjustment has to be made on the trigger before it will flash again. The blue light flashes indicating a signal has been received but no flash. That being said the setup works perfectly on my X-T2.