It was process of elimination. It didn't work well from the first time underwater, so I gradually eliminated all other possible issues until it could only be the ring light. Ultimately I was doing an A/B test with the same camera & optical cord on an Inon Z240 and the ring light. The Inon would always trigger and the kraken would not. The rep suggested my power wasn't high enough, so I repeated the test with 100% power from the on-camera triggering strobe. Same result.
In the end they sent me a new Kraken 3000 and the new one worked from the very first time out of the box. That kind of "proved in hindsight" that it was the first 3000 that was at fault.
Note - everything else worked perfectly on the first 3000, just that it would not reliably fire when triggered.