hi my name is josh i had the same problem check the terminals in the light socket they could be corroded that was my problem.... With a volt meter check to see if you have power to that socket. Check at the wire not the socket. If you find you have power, either the socket is corroded or broken. Also check the ground connection at that socket.
I would need to know what kind of car you have some cars have separate fuses and relays for both sides
have you checked your fuses?
Check your fuses. See owner's manual for location and assignment of fuses.
Defective flasher relay.
Check the flasher.
It depends on the vehicle but in general you would have one fuse for the signals and one fuse for the brake lights.
check the fuses and then check the flasher relays
The fuses for turn signals, I assume you are speaking on a Geo Prism, would be located to the left bottom(to the left of your left leg) front drivers side panel.
Bad switch in steering column? diffrent fuses.
Sounds like a relay, which should be near or in the fuse box.
The year, make and model would help but, check the fuses, the bulbs, the flasher relays and last but not least, the switch.
you might have a bad Flasher relay.