There is no ‘Sonos library’, Sonos points to a location where other data lives, but never has its own library, just a list of pointers.

Is there a possibility that certain songs are DRM protected? Sonos is incapable of getting the authorization tokens that Apple Music can, so Music would play the track, but Sonos wouldn’t. You can get around this if you have a Sonos speaker that is AirPlay 2 enabled, by just playing the Apple Music player, and sending the signal to your Sonos, and grouping if necessary. 

There’s also the possibility that certain songs may not be in the format that Sonos can ‘read’ when loading its library reference function. See the supported audio formats FAQ.

It might help others, including Sonos, investigate this if you were to provide particulars as to what tracks and what album is involved.