Play local files on mobile!

[oneofthegoths]

My Question or Issue

I have tried to put some local music files from my desktop in a playlist to listen to on mobile. I've gotten this to work several times but every time I add new songs they won't download eventhough I (1) added them to my mobile files and (2) I selected Download in the playlist.
I've looked through several forums to get help with this and have done every single fix possible from restarting my phone, making a new playlist, re-installing Spotify and everything else, but nothing seems to work. Any help if possible would be appreciated.

Plan

Premium

Country

The Netherlands

Device

Samsung Galaxy A70

Operating System

Android Pie (9)