Nylon stretches much more than steel after it's installed, so it's normal for a new set to drift flat over and over at first. The plain nylon trebles and the wound basses may also settle at different rates.
To help them settle:
- Tune to pitch right after installing the set.
- Gently stretch each string by lifting it slightly away from the fretboard along its length, then retune. Be gentle, especially with the trebles.
- Keep playing and retuning over the next few days. Each time, the strings drift a little less.
If a string keeps slipping well after the others have settled, check the knot at the bridge and the wraps at the tuning roller. A loose knot can look like a string that won't stop stretching.