What could be causing turning lights to not always turn on?

I have a 1999 Ford F150, owned it for about a year now, and recently (as in the past week) when I turn the indicator on, sometimes the lights won t flash. When they do work, the dash arrows flash and click like they re supposed to, but when they don t nothing happens. Hazards work fine. Just wondering if anyone might have a suggestion as to what needs to be replaced/repaired

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