Check for 6VDC on the lamp sockets. The driver board provides a ground to illuminate the lamps. Also could be bad sockets. The round bare wire braid going to each socket carries the 6VDC. The insulated wire on the socket goes to the driver board that provides the ground.

You can ground the insulated wire lug and see if the lamp lights. If yes could be incomplete circuit to the driver board caused by open somewhere or a bad connector on the driver board. If no and there is 6VDC then you have a socket or lamp problem.