Hello Jiffy,
If the lighter has been used for what it was intended for, you will find that the center connector of the sccket will have a build up of tobacco remains on it and requires cleaning, I do agree with Busterbus that the plugs are not the best and differ between one make and the other, consider fitting somthing like a mini DC plug and socket from a caravan dealer with a non reversable plug socket for more reliabilaty, even an old UK 3 pin round mini mains plug and socket would be more reliable than these items
Cheers Dutch RR