First place I would look is the connector in the tail. Make sure that all the connections are made and that the terminals are clean. A shot of Deoxit sure wouldn't hurt. Make sure you clean the bulb socket as well.
Try a new bulb. I had a turn signal where the bulb just wouldn't work in there even though it was a good bulb. Changed the bulb and everything was good.
Spray some WD-40 onto the switches. The front makes contact when the plunger comes out, and I have seen mine stick in in the "OFF" position, although it was the one on the clutch side. Rear could be dirty as well, with the dirt preventing motion of the little thingy that works the switch.