I am starting a new topic for getting local files working on mobile devices as it's just taken me a good 3 hours to get a few songs that were ALREADY ON MY PHONE to work in the spotify app.
It's good to see that you guys haven't fixed this, when there's multiple threads on here from 5 YEARS AGO with the same problem.
Basically, if you have music already on your phone, despite spotify having an "Import local music" option on the mobile app, you will need to transfer these files to your PC (Makes sense right?)
So first thing you obviously need to do is, connect your phone to your PC and get the songs off your phone and onto you PC.
Once they are on you PC, you need to go to "..." > Edit > Preferences > Local Files > Show Local files (Slide to on) (**NOTE** I found that putting in an option under "Add a source" gave me even more issues than letting spotify find the songs on it's own)
Once done, go to the left bar and find the playlist called "Local Files"
From here, add all the songs you want into a custom playlist. Here's where it gets absolutely ridiculous....
Now logout on both your PC & phone.
Open settings in your PC > Update & Security > Windows Security > Firewall & Network Protection. Once here, click all 3 options (Domain, Private & Public Network) and turn the firewall OFF.
Ensure both your mobile AND PC are on the same WIFI connection (**NOTE** your PC CANNOT be connected via Ethernet it MUST be WIFI)
Now, log back into both of your spotify accounts, search for the playlist you just created on your PC, on your phone. Check the "Download" or "Make available offline" box and the songs will begin to download.
Once you have all songs, don't forget to turn your firewall back on or your PC will be left vulnerable.
**Extra note** If you have an anti-virus software other than Windows Defender, you will need to google your anti-virus software information for turning off it's firewall AS WELL AS windows defenders.