You tune it, play two songs, and it's flat again. Tune it before bed, wake up, flat again. At this point most new ukulele owners conclude they bought a lemon, and most of them are wrong. What they bought is a perfectly normal ukulele wearing brand-new nylon strings — and nylon has a probation period.
Steel guitar strings stretch for a day or two. Nylon stretches for one to two weeks. It's not creeping out of the tuning pegs; the material itself is elongating under tension, like a very slow rubber band finding its length. Cheap uke, expensive uke, doesn't matter — the strings are the strings.
You can compress two weeks into a couple of days with pre-stretching. Gently — nylon takes a set, but it also snaps if you treat it like a bowstring.
1 – Tune to pitch with the tuner set to ukulele. Standard is g C E A — and yes, that little g is on top; the uke's re-entrant tuning surprises everyone once.
2 – Hook a finger under one string over the soundhole and pull it a centimeter or two away from the body. Hold three seconds, release, and retune. It'll be flat — that flatness is stretch you just took out in ten seconds instead of overnight.
3 – Repeat per string until a pull no longer drops the pitch much. Two or three rounds per session, a session or two per day, and by day two or three the uke holds tune overnight.
Slipping friction pegs. Many entry ukes have pegs held by a small screw on the button. If a peg turns back on its own — sometimes with an audible creak — tighten that screw a quarter turn. This is the single most common non-string culprit and it costs nothing to fix.
String wrap creep. Too many sloppy wraps around the peg keep settling. Two or three tidy wraps hold better than six overlapping ones; rewind if the coil looks like spaghetti.
Temperature and humidity. Nylon is more weather-sensitive than steel. A uke that lives near a radiator or rides in a hot car will wander daily no matter how settled the strings are. It'll also wander a few cents just from your hands warming the neck — literally. Don't chase that; anything within ±5 cents is in tune for all practical purposes, which is why the tuner glows at that point and lets you move on.
One more reassurance: the wound low-G string (if you use one) settles faster than the plain ones, so a uke where three strings hold and one drifts is normal mid-settling, not defective.