After around two years everything has worked fine with these strobes.
That's good to know David. My problem is that I (unusally for me) actually read the manual this time and know I have always aligned the marks as you suggest. I'm wondering if they may have another issue that is being masked by the known failure mode. Said another way, if S&S believes all the issues are caused by folks not aligning the marks they won't be looking for the real/other problem. It wouldn't be the first time a designers technical arrogance got in the way of solving a problem. Just a thought.
Cheers,
Steve