I have a Lumotec Oval sensor headlight wired to a Shimano generator hub (the newer version, I forget the model number). This setup has been working great for about a year, but this past weekend I swapped out my fork and now I'm having problems.
The light still works fine in "Auto" mode (where it's switched on and off by the built-in light sensor). But when I flip the switch to On, it sometimes works and sometimes doesn't. Fortunately I use Auto most of the time, but sometimes when it's rainy I want the headlight on even if it's not dark enough to trip the sensor.
Most generator light problems seem to be wiring or grounding issues, but I can't figure out why the Auto mode would avoid that -- or why the problem is intermittent.
Both the old and new forks are steel, and the light is mounted to the fork crown. The bracket is metal, which apparently causes grounding problems for some people with the Shimano hub, but I've never had an issue. The bracket is also painted, so I'm not sure if it's making an electrical contact or not.