I have a Speedlite 320EX and am experience the same issue I have seen others discussing here with the 600 speedlight.
The problem is the unit doesn't turn on with batteries in it.
If I open the battery compartment and then hold it closed without sliding it forward then the flash works. However, as soon as I slide it a bit forward to close it the flash stops working.
I'm guessing there is an issue with the connection to the batteries but this behavior is very bizarre.
I took a bunch of photos with the flash earlier in the day and then later when I tried to take more the flash didn't work. I thought maybe I had killed the batteries so I put fresh ones in and the flash still did not work. After reading about the issues with the 600 I experimented with my 320 and was able to determine that it worked if I held the battery door but not otherwise.
Anyone have the same experience? Any idea how to fix it?