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seager
tl:dr - commuting for 3+ hours a day, have given up on dynamos. Sorry. Not sorry. With caveats.
This likely won't be all that helpful, but I've had intermittent problem with rear B&M lights not working.
First time, after three 'not working' issues, sent it back to Spa Cycles and they sent it, after testing, on to B&M - and no-one could find a fault. Swapped the rear light out (twice - the new one Spa kindly replaced FOC then another brand I bought) and had the same intermittent issues. After an angry few moments during a very cold, dark morning I swapped the connections around (the ones from the B&M front lamp that lead to the back) from sheer anger and the rear lamp came on.
This has worked a few times when the rear lamp had decided to take a break from lighting up. I have also undone the connections and wiped them with a dampened thumb and forefinger before reconnecting to the correct polarity(?) and that also encouraged things along.
The other snag that also affected the back lamp appeared to be the very thin cable. Mine ran along the top tube with the rear brake line and sometimes seemed to get compressed (is the only way can describe it) and so the light failed.
In addition, the totally awful roads (ha! cart-tracks more like) I have to commute on rendered the (Chrina) rim worn-out after 14 months riding, so faced with replacing that (ok, cheap but still a consumable, plus my time to re-thread a new one, plus have done 10k miles so the dynamo would have need a service soon) have gone back to rechargables; cheap, bright, last two hours (all I need one way.)
Don't hate me.