I recently encountered a similar problem: there were about 50 local files that I synced with my phone back in January, but for some reason they became greyed out all of a sudden last week. I tried re-syncing today but only about 20 of the 50 went through. I know it wasn't a firewall problem because my Spotify desktop recognized my phone as a connected device (and also because of the 20 songs that were able to re-sync successfully).
I spent the next 2 hours trying to find out why the rest of my local files weren't re-syncing. I tried switching the folders of my local files on my computer, editing the metadata of the songs, creating new playlists, reinstalling Spotify... still did not work.
But then I finally figured it out! Long story short, here's what I did:
1.) Open Spotify on computer desktop and mobile phone. Make sure that the phone is connected to the same Wi-Fi as the computer and that the phone is recognized as a device on Spotify desktop.
2.) On Spotify desktop, create a new playlist named "Local" and add all local files into it (by dragging and dropping).
3.) On the phone, go to the Playlists tab and check that the newly created "Local" playlist shows up.
4.) Here's the thing that got me: Make sure that your local files are not in any other Downloaded/Offline playlist on your phone!!! I noticed that my 30 local files that failed to re-sync were part of another playlist I had created back in January. This playlist, which contained both local and streaming files, was set to Downloaded on my phone; as a result, all songs -- even the greyed-out local ones that couldn't be played -- had the little green arrow/circle next to them. I'm thinking this caused the program to fail to recognize that the local files in that playlist needed re-syncing.
5.) Remove those old playlists off of Downloaded/Offline and make sure the green arrow/circle no longer shows up next to each song.
6.) Restart Spotify on the phone. Go to the newly created "Local" playlist and set it to Downloaded/Offline.
7.) All local files should start syncing now! Once finished, other playlists can be set to Downloaded/Offline again.
Even when I download a playlist, the files on my Mac won't transfer over as local. The gray dowload icon turns up in the desktop app so I think there's been an update that got rid of functionality for downloading local files?