If you have the Yamaha aux lights like most of us do, you will have a good chance of getting water in the turn siginal housing every time you wash the bike. On mine I have to take the turn siginal apart about once a year to clear some rust out of the bulb socket or I have the same condition you decribe. The bulb is fine but wont make a good contact with the socket. Try some 200 or 300 grit wet and dry sandpaper, rolled up to fit in the socket, and twist it around a few times, then clean the small dots in the bottom of the socket with a small flat blade screwdriver. Should take care of the problem. Good luck.