since they are vintage style take off the cover and see if the gear is wearing badly, happens sometimes on cheaper tuners, i had a guitar the other week where one tuner gear had lost half its teeth - that was on a really cheap guitar though.
maybe its some build up of cr@p so give it a clean out, If the gears look fine put some oil on and give them a good turning - get a string winder on it and gives it loads of turns to work it in