Local files not appearing on iPhone AFTER playlist downlaoded.

[The_Rhino]

Plan

Premium

Country

Australia

Device

iPhone 6 and Windows Desktop

Operating System

iOS 12 and Windows 10

My Issue

I have downloaded many local files from my computer to my phone in the past. Each one, I moved from the Local Files playlist on my PC to their own playlist, clicked 'download', waited quite a while and eventually the song appeared on my iPhone. 

However, recently I tried syncing another song that I just added to my PC. It worked on my PC perfecetly fine, being an MP3 file, and after moving it to a new playlist and clicking 'download', i waited for it to get added to my phone. 5 days later and it's still not there. Both devices are definetly on the same wifi network so I downloaded the empty playlist on my phone aswell [on my PC, the playlist has the song in it but on my phone it was empty]. Still, even more days later, the local file hasn't synced with my phone. What can I do?