I suggest a phone call to Peter White. IIRC there is something about some B&M lights that has to do with grounding. If used with a Shimano dynamo hub which grounds itself through the hub axle then SOME B&M lights require that a wire be moved or removed for proper operation. This is not required for use with Schmidt SON dynamo hubs.

I looked on the B&M and Peter White web sites and could not find the info covering this so am possibly mistaken. Try running the light while it is not bolted to the bike, held in your hand with only the wiring attached to the hub. If it works correctly then you may have a grounding problem. If not then you may have a defective light.