Why are some of music files not being added to my iCloud Music Library?
Solution 1:
There's an option in the menus to force an update of the iCloud Music Library: File > Library > Update iCloud Music Library.
This caused a spinner to appear in the upper right-hand corner of iTunes and, once it disappeared, all my formerly-unavailable tracks were now available in my iCloud Music Library for streaming from my other devices.
There was no need to log out and back in to my iTunes account.