Local files are not linking to Spotify's database entry

[ACAx1985]

Windows 10, latest update

At some point in the past, I was able to set a Local Folder and put files into it. Spotify would then recognize many (not all) of these files and play the local file as a result.

For example, if I had a local file named "Led Zeppelin - Stairway to Heaven.mp3", with proper ID3 tags exactly set to the same as Spotify's database entry for this song, it would link the file and play my local file instead. 

This has stopped working at some point, and now NO LOCAL FILES sync in this way.

How do I re-enable this functionality?

Thanks!