I suddenly have the same problem a couple others have mentioned. I use an ST-E3-RT to link three 600EX-RT flashes with a 5D MIII. Recently the link started randomly dropping on all the flashes (after some random amount of time from 1.5 minutes to about 4.5 minutes). The only way to fix it is to turn everything off and reset each flash to slave. Not good during a photo portrait session since it happens multiple times.
I have tried a ton of troubleshooting. I used to shoot weddings so I have 2 cameras and two transmitters. I've tried all combinations and the link drops regardless of the camera or transmitter and even if the transmitter isn't on a camera (hand held for test). Both transmitters drop. I tried several different channels and auto - no change. I tried each flash by itself to see if is something with 3 flashes - no. Same problem with only one. I have 4 flashes and I tried the 4th just to see - same problem.
One last piece of info. When the link on the flash drops, the light on the transmitter still shows green (so it thinks it is linked). Turning on and off the transmitter doesn't re-sync the flashes.
This seems crazy. It can't be the camera (I even tried it with a new R6 that my husband got - same issue), because it happens when not connected to the camera. Why would both transmitters go bad? Why would all 4 flashes go bad. I don't know what else to try.
Any ideas, I'm pretty desperate at this point.
Hard to say exactly what might be causing the disconnect, but since the common element would be the connection between the units regardless of transmitter or flash as transmitter it might be a matter of settings or interference. There are a couple things we can check.
If you haven;t done so already I recommend clearing the settings entirely on a couple pieces to make a brand new connection. When doing this I also recommend changing the channel and wireless radio ID to something away from the default, that way we might shift the signal away from potential interference from other sources. If the connection stays stable then I would suggest adding each additional piece to the system with time in between to confirm that one of them is not reintroducing the issue.
Thank you for the suggestions, however unfortunately this didn't work. I cleared all function settings on 2 flashes and a receiver and I changed both the group and the channel (I scanned to find a good channel). Flashes still unlink after a short time. I will have to call Canon in the morning to see what they say. Very frustrating. P.S. - although I use these in my home studio, I also had it happen twice outdoors where there was no wi-fi around.
I have had the exact same problem with my 430ex iii-rt units. They use to work flawlessly. I tried resetting cameras and flashes. I tried multiple combinations to identify the problem. Nothing has worked. If you do discover a fix, please let me know. If I discover a fix, I will let you know. In the meantime, it's frustrating as it ruins the flow in a photoshoot, which you already know.
I am also having this problem. I've acquired my (3) 600 EX-RTs over time and from numerous sources. This has been a problem in the past, but has seemingly gotten worse lately. They're effectively unusable now for professional work as i look foolish constantly power cycling the units to get them to reconnect. I'd love to know if there's a solution. Thanks for posting this.
PS: I also use an ST-E3-RT to link three 600EX-RT flashes with a 5D MIII
I have sent the flashes and the transmitters to Canon. Estimate of $195 shipped to fix each flash and $170 shipped to fix each transmitter. However this is just an estimate. I read someone else had this problem and they couldn't find anything wrong. Hopefully they will be able to fix it and it won't cost as much as the estimate because that's very expensive. Agree on looking foolish and that's why I sent them to see if they can be fixed. I will keep you updated when I get anything back on the repair.
At that cost, it might be cheaper and more effective to get a godox transmitter and receiver and use the flashes as dumb units sitting atop them. I'm checking with my local camera shop for when the canon rep is coming to town.
There are a few threads about this issue here. WiFi interference with the 2.4 GHz these flashes use seems to be the agreed-upon culprit. This happened to me with 2 different R6s and 2 different 600EX II-RTs. Sent to Canon to diagnose, but no luck.
Interestingly enough, this hasn't happened so far with EL-1 and version 2 ST-E3-RT.