(Kudos to Jay-eye of Birmingham, UK for this tip, posted on the forum at Ukulele Cosmos.)
A lot of folk have problems with the friction tuners on their ukulele, specifically when it comes to fine tuning.
You know the symptoms to this problem. You find a string a little flat, give a slight tweak to the peg and it goes too sharp, adjust a bit for that and now it’s too flat. Now back to sharp. This seems to go on forever, especially if you’re trying to retune while people are waiting for you, like at a performance or in a uke-club setting.
Either blame gets assigned to the tuners themselves or the person will think their own tuning skills are sub-par. But sometimes the problem could be as simple as the string sticking a bit on the nut. This causes the string to be stuck on a note even though you’re tightening or loosening it with the peg. When the string “comes unstuck,” it adjusts to the new tension and you go either sharp or flat.
Jay-eye remembered an old fiddle trick and rubbed a fine pencil point into each string groove on the nut. That leaves just enough graphite to lubricate the string and stop its sticking.
It works just as well on a ukulele’s nylon strings as it does for the metal strings on a fiddle. Also this might be the problem even if you’re using geared tuners. Stuck is stuck!