I had this exact issue occur when I plugged in my cheapie Chinese LED H7 headlight bulbs. Thankfully, I recalled the polarity issue and reversed the plug, upon which the lights worked fine. Mine were un-marked for polarity.
Note that it is extremely easy to get the H7 blub in, out of position. They don't sit flat and will then not give good light, especially with LED capsules. IIRC, the tab is about 7 o'clock on both housings. They only fit correctly in one spot. Get them off and they will be angled and give poor performance thru the lens.