All I can think of is that there is some kind of mismatch between bulbs and holders. As Stephen says, the standard bulb has two filaments with a common earth to the cap. So either filament will illuminate depending on which contact on the bulb is connected to the power.
A bulb doesn't have a polarity. It just needs a positive supply to one end of the filament and negative to the other. The earth end is set at negative by design, so the supply to the contacts has to be positive. Obviously earthing both ends won't work.
Decades ago there were bulbs that looked outwardly like the larger 5/21 watt bulbs and had two contacts, but only had one filament and no earth. So one contact had the supply voltage and the other was independently earthed. If you plugged one of those into a dual-filament socket it wouldn't work because there would be two positive connections and no earth.
Could it be that some very modern LED bulbs work a bit like that and need a special socket?
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