I only check once in a while (even though I could use it freely), but first off I listen to a lot of live music so there is that, most is not there or freely traded anyway.

But I can think of a good example recently: I wanted to hear Burning Spear's Mistress Music, with the track Fly me to the moon in particular. It is not there.

Not a huge artist, but also not unknown either: https://www.discogs.com/master/108163-Burning-Spear-Mistress-Music

It is in my music server now (somewhere I have the CD, but who knows where).

It seems to happen to me more often than not.