I've had this problem in the past but it has been happening a lot more this week. I have some folders with .mp3s in them and I've been playing them just fine for months now with Spotify, but sometimes they will become unavailable randomly when I open up Spotify. Sometimes a restart of the program will fix it but some files are just permanently unavailable when they played just fine the day before. Sometimes just making a copy of the .mp3 in the folder fixes it or I have to delete and re-add my entire music folder to get my local files to show up.
-All files are .mp3s
-The files have never been moved, so that's not the issue
-The files have been played before in Spotify
-The artists don't exist on Spotify, so there isn't an issue of mix up between a local file and a Spotify one
I've been a premium subscriber for years now and, while I love the program, it hasn't been great with local files.