Just move the bulb to a different socket and see if it lights. If so, then there is an issue with the socket. There could be one of two issues, both caused by corrosion. The first is corrosion in the bulb contact center pad. You need to be careful using a wire brush on that because it is not made of the heaviest material and the brush could tear it out. A pencil erasure actually works very well on them. The other contact is on the side and depending on the socket, may or may not be an issue. If it is the type with the metal threads, that type is normally not affected by some corrosion due to the firm contact between it and the bulb. If it is the lower cost plastic socket with the metal strip, then make sure that is also clean and make sure that both contacts are bent out toward the bulb and not pressed into the socket.
The second issue is that one or more of the contacts have corroded and lost there connection with the wires in the socket. In that case, the only option that you have are to either leave the bulb out or replace the socket. Lets hope that the only issues are a few burnt out bulbs or at worst, a dirty or bent contact.