I have mp3 files downloaded to my Mac that I would like on my Spotify. I have added local files in the past but now when I try to add them it won't sync. I have the source file turned on and it is the correct file that I want. Other songs in the file are there, but not these.
You mean you can't see the files on Spotify on your Mac or they don't sync on a mobile device from the Mac?
Could you first perform a clean reinstall of the app on your Mac? Here is how to do one. It's also a good idea to update or install the latest version of QuickTime from here.
If that doesn't resolve the issue, it could be possible, that the files you want to import are DRM-protected. Can you make sure you're not trying to import such files, more info on that can be found here?
Keep us posted,
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