So I'm in the middle of shooting (just practice, there's no model here today, thankfully!) When suddenly, my flashgun stops firing in optical wireless mode. It's been working fine up until now. I didn't change a thing, didn't touch it up on its tripod - it just stopped responding. I put fresh batteries in, I checked that I was still on channel 1 on both camera and flash, I have everything in full manual, gun on wireless slave mode. I moved it right up to my camera to factor out lost line-of-sight. I even mounted it on my camera again and it does still fire just fine, so the bulb's not toast. Can anyone think of anything I might have forgotten to check?