Pull them off the headstock, taking some photos so you know how they go back on. Clean all the metal parts and don't lubricate, no oil. Check the bits which seat the tuners in the hole. While they are off, give the head stock a clean up as well, being careful of transfer type labels. Usually you need a screwdriver and a 10mm spanner and a jar for the parts, don't use an adjustable spanner.
While the tuner knobs are off, sand off any rough join marks and give them a polish.
Then re-assemble.
Friction tuners are very simple, they slip when there is not enough friction, so you are looking to get rid of causes of low friction. Also they need to be seated properly, so the seating hardware has to be right as well.
Note that there is a difference between a spanner and a spanner in the USA and Australia. In Australia a spanner is what is called a wrench in the USA. It is not adjustable and is made to exactly fit the nut. In Australia the adjustable spanner which was made famous by the Crescent company is called a Crescent or an adjustable spanner. It can have a sloppy grip and bur the nut, so don't use it on your ukuleles. Many of the tuning machines I have on instruments use a 10mm nut to hold the tuning machine in place, it needs to be removed to take the locating hardware off the headstock.