Did they use one of the 6" long testers? There maybe an issue with those.
They have LEDs in them which take a minute amount of power to light. Its possible for them to light up and say the socket/connection is fine but the connection is not good enough to power a proper bulb.
Seen it first hand. Tester said the car was good and it also said the caravan was good. But when connected nothing. Remove the bulb and tested the socket 12v showed on the meter. So 12v is reaching the bulbs but no lights?
After some head scratching i fitted the bulb and tested the voltage, it was way below 1volt. Nowhere near enough to light the bulb.
There was a break in the wire inside the car but enough contact for a voltage to show, not enough contact to actually power a bulb though. Connected it properly and all the lights worked as they should.
Added some resistors to my tester so that it draws more power.
all fixed now by a local caravan maintance man the 7 pin plug on the caravan was changed and and wiring on the rear lights changed and new bulbs fitted and 2 fuses changes now our caravan lights up like a christmas tree and works working happy days
no not really only 50 quid and all works amazing and now in 4 weeks off we go down to cornwall...having a van we have our own toilet and shower facilites we dont have to relay on site facilities and share with othwr campers