Sync local songs on mobile doesn't work anymore

[user-removed]

Plan

Premium

Country

Germany

Device

Samsung Galaxy A3 2017

Operating System

Android 8.0, Windows 10

My Question or Issue

 About a month ago I already synced a local song from my PC  on my phone, I added the file to a playlist and made it offline available on the phone. It worked fine, but now I tried to do the same thing with two other songs and it ain't sync. I googled and some people said I should check if I'm connected to the same WiFi (which I am) or if my firewall allows spotify (it also does). 

Anything else to check or a new setting I have to enable?