Buick Regal turn signal

[rafsef]

My left turn signal will not flash. I replaced both bulbs in the rear tail light. They light up but will not flash. Checked the front bulb and it is fine, The right turn signal works

 

[salvageV6]

Check the side marker bulbs, note how everything flashes on the good side and check the rest of the bulbs on the bad side.
Make sure the Hazard switch and all the lamps work with that switch as well cycle it a few times then try the turn signals again.