Each nostril insert has two concentric cones. I found that the inner cone when it gets worn can retract to a position about 3/16" farther from the nose, leaving the outer cone without sufficient support to make a good seal with the nostril. As a temporary fix I could pop the inner cone back to its normal position.
The inner cone was bi-stable: it was either correct or retracted all the way; it was not stable at any intermediate position. This is good because you don’t have to worry about whether the inner cone is slightly out of position.
I did not experiment with how many days I could keep using the pillow with the temporary fix. I immediately ordered a replacement pillow for $20. (I feel getting a few months use out of a $20 pillow is excellent value.)