p.1 #1 · How To Tighten the Hotshoe/Foot on Your Canon Speedlite
I had some issues with a 580EX II hotshoe foot coming loose. It became so loose over time that the flash started losing communication with the camera. Symptoms included chronic E-TTL overexposure and lack of in-camera menu control.
The fix was simple and took about 45 minutes. It would have taken 20 minutes but I got into a fight with the quick-release button spring
p.1 #2 · How To Tighten the Hotshoe/Foot on Your Canon Speedlite
I have never had that issue with my flash units but have with a Canon 5D. The Camera hotshoe would come loose.Not enough to feel but enough to not work 100% of the time. Same things went worng as you describe. For those with flash issues that is something else to check. Easy to tighten by removing a slide in plate on top. Takes 2-3 minutes, or less after you learn how to lift and slide the plate out of position.Sorry I don't have a video showing how but I am sure many are available.