Well I set up for radio transmission with two 600EX-RT's and put one on camera as master, and one off camera as slave. I am firing in automatic ETTL mode. I set them up, they are linked, same channel and frequency, but the master unit will not fire, only the slave unit fires. In fact, I cannot even set the master unit to fire, because the ON or OFF button just doesn't work. Where that icon goes in the LCD screen, right next to it is an exclamation point. Does anybody know what I'm doing wrong? I've read and reread the manual and followed all the steps and it's not working. Thanks.