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)â€”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 askâ€”are you by any chance running Catalina? There are lots of (ongoing) audio/DAW issues with it, even with Apple's own audio softwareâ€”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 upâ€”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 Loopsâ€”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..