During a wedding shoot one day I noticed that my flash was only firing occasionally. After the initial panic, I quickly figured out that if I held the flash with a little counter-clockwise tension, then the flash worked fine. At the first break, I tried my backup camera and it was doing the same thing. When I got home, I checked it against my daughter’s 20D and it worked fine, and it also seemed that the hot shoe on my cameras was loose compared to hers.Another symptom of this is that the flash will switch from E-TTL mode into TTL mode and the exposure will usually be very overexposed.