Canon's wireless flash control operates on 2.4 Ghz. which is a VERY heavily used part of the RF spectrum. Although the IEEE provides more detail about the spectrum, Wikipedia is sufficient to provide an idea of the level of pollution and crowding in this part of the spectrum: https://en.wikipedia.org/wiki/List_of_2.4_GHz_radio_use
Unfortunately this is a problem for many devices and it is one of those undesired side-effects of using political appointees instead of scientists and engineers in the various world regulatory agencies. Courtesy of a new garage door opener I now have to drive my Cadillac ATS a few hundred feet down the driveway so that the TPMS system will properly reset when I change between summer and winter wheel/tire sets because of co-interference between the door opener and tire pressure monitoring system. It is a problem that has continued to get worse instead of better especially because it is coupled with a rapid increase in the number of unintentional RF radiators (i.e. switching power supplies, digital controllers, etc) which produce RF pollution as a byproduct of their normal operation. In addition to specific interference between two devices operating on exactly the same frequency there is the more general problem that the sensitivity of receiving devices is reduced by the number of transmitting devices in the same general location and frequency range. And while your flash is frequency hopping to try and find a clear spot other devices are doing the same so results during a single event are likely to be somewhat random as a formerly clear spot is suddenly being shared with other devices that have chosen your frequency as a possible solution.
Wireless flash control is so time critical that typical measures used to handle communications interference (like multiple retries) are unworkable. At this point for a critical event I wouldn't count on a wireless flash system being practical unless you have a chance to pretest in that location under the exact conditions that you will shoot and that isn't practical for a wedding reception or similar event where entertainment AND guests are likely to bring in stuff that creates a problem. Think of how many guests are carrying around Bluetooth enabled devices which are operating on the same slice of spectrum as your wireless flash. The old infrared system was limited and also experienced some interference but it may well be more reliable in many venues than the more modern RF system.
You can improve your chances of success by locating the controller and slaves as close together as possible. The smart controllers in most wireless devices will transmit at their maximum design power but this will drop as battery voltage drops so it will also help to keep the voltage supply to the various components as close to maximum charge as possible. Fresh batteries to start and changing during an event can help make the system more robust.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
