Like john said one bad bulb (or connection) will keep it from blinking.
Turn on the left blinker and look at the bulbs to see if one is out.If one is bad or not on, replace it or make sure the socket is clean by spinning some emory cloth or fine sand paper in the socket and on the bulb shaft (if the bulb is still good).
This and a bad ground is 99% of the problems with on side out.
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