Why does one set of G9 led bulbs work and others do not?

I have 2 x identical ceiling lights (each with 5 bulbs) and recently decided to change the G9 led bulbs from bright white to cool white however only 1 of the lights worked. All 5 bulbs failed to light up in one but not the other. I changed back to the old bulbs – all lit up. I swapped the new bulbs from the lamp that was working but still nothing.
Both lamps are non dimmable and the switches are simple on/off type.
I am completely baffled as I can see no reason for this – can anyone suggest why this might be?
Yes, I have tried swapping all bulbs around but the Integral LED G9 bulb refuses to work in the one light fitting. It is a complete mystery- totally baffled.