More often than not I find exterior lighting problems to be on account of a ground that has incurred high resistance, or a found wire that was detached for some reason, and either not reattached, or attached in a way to causes high resistance to ground. Before buying bulbs I would check for a poor ground via voltage drop measurement, or swap the bulb with a working bulb. If the working bulb is not working in the socket where the lighting is not working it is a wiring/ground/socket issue - or if you are unlucky it could be a combination of those factors. I use a test light to make certain the socket is getting power before looking at anything else.