OK, then. Not knowing the age, brand or quality or this particular instrument, I will give you the generic fix. Remove all the strings and take both strips of tuners off. If they are made with simple peened-end tuning knob shafts, check the shafts for loose-fit in the little ears that hold them engaged to the ring gear that drives the nylon. Too much slop will make them bind and you can do very little to fix that. If you find no large amounts of play in those little retainers, simply wax (a candle works well) the ends of the nylons and the worm gear. If your knob shafts have screws that retain them, it gets a little harder to do. You will have to take the screws out, clean them, and put them back it with LokTite to keep them from changing settings. Adjust them while the LokTite is still fresh (no need to have the strings on yet) until they all have a reasonable amount of friction and leave it overnight. Still wax the nylon and gears as they will love you for it.