If the other lamps work in the fixtures you at least know the fixtures are grounded. Beyond that you'll need to do some poking around with a test light or multi-meter. The film type contacts sheet in the fixtures are prone to suffer abuse--at the lamp sockets and the folded over section where the wire harness connector attaches. The lamp sockets themselves can have burned or bent contact tangs. Check that the proper sockets are installed--most spots use a two contact socket--some use a three contact spot (double filament bulb)--you can see it on the film sheet. -- Dave