It could be a buld is burned out, or one of the bulbs in the socket is not making good contact as it could be corroded, causing a bad ground. It could also be a bad or poor ground in the wiring, or could be the flasher relay. Also sometimes the blinker mechenisum in the steering columb that the blinker arm is attached to, goes bad.
It can be time consuming finding the problem, around here they get 85.00 Hr in the shops to fix such problems. I'd start with the bulbs.
If the turn signal arm does not give you a positive up, down, and center click, there is a spring in there that likes to brake, and sometimes the plastic piece the arm screws to likes to crack, causing such problems.
Good luck........