Canon 600EX-RT Question

This is also posted on the 5D forum.

Kinda embarrassed to ask this question but here goes.

I have been using 2 600EX-RT's for about a year now with no issues at all. I always have the same one set to master and slave. Just today, I tried using the slave mounted on a camera for just basic flash use. But it doesn't work.

I have cleared the setting, set the unit to master, doesn't work, set it to neither master or slave, doesn't work. I test the flash by pressing the test fire button and it works fine. It isn't low on battery.

Weird, so I put it back as slave, the way I had it before and it fires just fine as a slave. What am I missing? Browsed through the manual and no help.

It's gotta be something very simple....please help. Thanks.