I am a wedding photographer and I have shot 26 weddings this year with three 600ex-rt flashes and for the most part am very happy with them. However, there have been two occasions where there has been an area about 20-30 feet in diamater that the master goes all screwball and the link goes in and out. I was shooting a wedding last night and my assistant also had three flashes and hers did the same thing in the same "dead zone" We were both on different channels so we were not interfering with each other. I did everything I could think of like change batteries turn them on and off and I even did a channel scan to see which one had them most strength. Still there was that zone where the flash had issues and the link would go in and out. It was in the middle of a large room with nothing that I could see that would cause issues. It wasn't next to anything else electrical that I could see. The only thing was a florescent fixture about 30 feet in the air. Once I was out of the dead zone everything was fine. Unfortunately it was the dance floor so it made shooting the reception harder. I even tried to stand over by the DJ were I figure there would be the most interference and I had no issues. The dead zone was in the middle of nothing. Anyone have any insight?