I mentioned this on Matrix but thought I'd add a post here to make it official: I realized that songs in some of my playlists weren't playing or even loading because the libraries that contained some of the tracks were deleted, but it was hard to figure this out because there wasn't any indication why. This has happened in other contexts too—a track wouldn't play and I'd just sit there scratching my head wondering why. As I discovered, the act of following a library is really pivotal to the user experience on Funkwhale, but it seems like the importance of this action isn't really indicated through the user interface!
As a way of helping users fix this problem, I'd like to suggest that playlist pages (i.e. the ones that display all the tracks in a playlist) and "now playing" pages (i.e. the page that shows which track from your queue is currently playing) show some kind of indication which library you should follow to help the track load properly.
For example, the "now playing" page already shows an error message when a track won't load. Why not extend that message slightly, with something like "To help this track load, try following X library." Further, the playlist page could show a list of all the libraries a user would have to follow to play all the tracks in the playlist.