More than likely the rods that go from the door lock actuator to the lock by the door handle are sticking in the plastic clips. They are originally lubed with i believe white lithium grease, they get gummed up with dirt and crud over the years.
Take the door panel off and look for the white clips that the rods snap into and remove them. Clean them up and relube with a light coat of white lithium grease. If you've never removed the door panel, I'm sure there's info on how to do it on here or in a YouTube video. Good luck!
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