Hi Alex -
This seems to be a pretty common (and long-running) issue, though it looks like you've already tried all the usual suggestions I'd make. Deleting the file ~/Music/Audio Music Apps/Databases/LoopsDatabaseV09.db, relaunching GB, and re-indexing is the action most frequently recommended by Logic/GB experts (the same issue occurs with Logic as well)—it's the solution that seems to work best most of the time.
Were any other GB/system changes/updates done just before the problem started occurring? I have to ask—are you by any chance running Catalina? There are lots of (ongoing) audio/DAW issues with it, even with Apple's own audio software—it that's the case, can you restore/backup to Mojave?
You seemed to imply that files that were imported previously and working fine and then all of a sudden stopped showing up—is that the case, or is it just happening with newly-imported files?
It sounds like you're experienced with troubleshooting in general, but since you said you're new to GB it may be worth mentioning that one user who was having a similar problem found that he had category/filtering options (at the top of the Loop Browser) engaged unintentionally, and that was what was preventing him from seeing the new Loops—I guess it's worth checking out..
Additional standard troubleshooting steps (you're probably already familiar with these and have tried them)..
- Hard shutdown | restart
- Try running GB from a different User account
- Apple's official suggestion, FWIW (again, it looks like you've tried this): https://support.apple.com/en-us/HT204198
A quick search turned up these additional suggestions..
- [Quote] When I try to add wav files as loops by dragging them onto the Loop Browser, they will be just installed in the folder ~/Music/Audio Music Apps/Untagged Loops/User Loops/, but do not show in the Loop Browser. Dragging .caf loops into the Loop Browser still works as expected and add them to the Loop Browser. To turn a .wav file into a loop I add it to a track and select it and use the command "File > Add region to the the Loop Library". [Unquote]
- [Quote] GarageBand only imports MP3 files which have the extension in lower case: ".mp3". If you try to import MP3 files ... with the extension in upper case: ".MP3", GarageBand will refuse to import those kinds of MP3 audio. In this case, change the MP3 file extension to lower case in order to import MP3 to GarageBand. [Unquote]
- [Quote] GarageBand only supports those MP3 audio files which have a sample rate of 44.1kHz and a bit depth of 16bit. In this case, it will be necessary to convert the MP3 files to GarageBand's more compatible AIFF file format. [Unquote]
I can't speak to the effectiveness of these last three suggestions, but if nothing else is working, and if any of the scenarios fit, they might be worth a try..