YONGNUO 600EX RT 2 - AF ASSIST LIGHT LOSING FOCUS

Can you see the AF assist light projected on a white wall?
And when you look through the viewfinder of the camera, is the light visible?

Hi Bergstrom.
Just a thought, are you using single point or zone AF? In single point if the only af point is not coinciding with an AF assist spot then focus is doomed to fail, if you are using zone then at least one point should find an AF assist spot.
I suppose after time the LEDs that project the AF assist spots might get weak, or are you frequently using the flash in a dusty environment, perhaps the inside of the red cover is dusty?

I guess it is problems like this that keep mission critical (eg wedding) photographers using genuine Canon accessories, they cannot afford to be hit with problems like this!
I have a pair of these flashes and the YN-E3-RT to go with them, after a few months and even fewer uses I noticed that one of the led cover lenses (clear plastic over the blue connection status led) had vanished from one of the flashes, annoying but no big deal, it still works.
I hope you get to the bottom of your issue.

Cheers, Graham.

Hi Bergstrom.
I might be mistaken but doesn’t AI Servo disable the assist beam anyway? So many things to remember about how these things interact!
I suspect near darkness is always going to be tricky to get a focus lock.
To be honest, if I didn’t need the assist beam I would probably use AI Servo anyway, I have found it can help if I have been exerting myself and am swaying to my heartbeat!

Cheers, Graham.