Ok. So I have checked the show local files switch. I have added the source. I have even tried making a new playlist. However, none of my local files show up. Do I have to import them somehow? I have searched for help on Spotify and it just redirects me to the page where it says to turn on show local files. I've already done that. Can someone please tell me what is wrong? These are English songs, they do not have special characters so I know that isn't the issue.
If your local files still don't show after trying some troubleshooting steps you might need to perform a clean reinstall of Spotify on your Windows device. This will remove corrupted data that could be causing the issue.
In case you see no difference after the clean reinstall, check your hosts file for entries that need to be deleted. Follow the steps described here on how you can do that.
Let us know how you get on! We'll be here in case you need any further help with this.