I've had that happen a couple of times recently with my 1st gen EM-5. My last several dives, even just yesterday, I had no issues as I tried to be certain to seat the strobe well. My guess is that the less-than-robust electrical connectors in either the strobe or the shoe have been blocked by dust, much like mobiles that run into this problem. The spaces involved are tiny, however, and even a small bit of plastic is difficult to insert when trying to clean these contacts. I suppose the take-home message is to either 1) keep the flash on the camera at all times, or 2) use the dust cap. Fingers crossed this won't happen again!