Cabin Lights not working

[26bertram]

I have two cabin lights that don't work. Fuses are good. When I turn on the switch and test it with the volt meter it shows power coming from the two prongs that are supposed make contact with the bulb. I believe that somehow I am not getting a good ground with the bulb & brass fixtures(2). The bulb sits in one and another pieces screws into the light fixture holding it in place. I have moved the bulb and fixtures around but no lights. Any help would be appreciated. It is a 1983 Bertram, 26ft.