Well, I hope I can explain this adequately enough. Basically, the flash is not seeded onto the camera right. Now before you deny this, let me explain.
My cameras: Nikon D750, D610 (x2)
If you look at your camera hot shoe, you will notice a small hole in the front of the shoe (if you are looking at the shoe from the back of the camera).
Now turn your YN-560-TX over and look at it. Where the small hole is in the hot shoe, there will be a pin that pops out as you twist the flash controller unit's wheel (the wheel that fastens the FCunit to the camera)
This pin MUST go into the hole on the camera or else it will not fire the flash units more than once. So how I accomplished this task was to unscrew the wheel on the 560-TX until the pin was sticking out the furthest.
Here is the tricky part: you must slowly slide the FCunit onto the hot shoe and "listen" for the soft click. When you hear it, that's where it needs to be.
Apparently, the flash can be shoved forward too far, and its quite easy to accomplish this.
Is this an error on Yongnuo's part? I think maybe it is.
I was just getting ready to return my units when I came across this in a Google search and had hope that maybe there was a fix....unfortunately, it's not working for me. Tested with a Fuji X-T1, I can see the pin & hear the click, but it doesn't change anything for me. I've ben slowing listening and adjusting the position of the TX, but the flash still fires one time and one time only.