Some local files aren't syncing

[user-removed]

Plan

Premium

Country

 USA

Device

Moto Z2 Force

Operating System

Android Oreo

My Question or Issue

 Sometime around 2015 Spotify had a lot of problems syncing local files. I almost left the platform/cancelled Premium then, but the team seemed to fix the problem and I didn't have any issues syncing local files for a few years. However, the problem seems to have returned. Some local files sync with no problems. I add them to playlists in the desktop app, download the app on my phone, and it picks up the files via wifi. Easy. I just noticed today, though, that certain files just won't sync no matter what I do. Move them to a different folder, delete them, re-add them, log out, log back in, whatever I do they won't download when I download a playlist to my phone. Why is this issue back? Why does this only happen to certain songs? Are there some rules that I'm not aware of? The help page says "illegally downloaded files are not allowed" but these aren't illegally downloaded files—they are recordings I made myself from vinyl records that I own legally. Is there a way to resolve this? Is it just a bug that's being worked on? I'm pretty frustrated that after many years as a loyal Premium subscriber this is happening again, but if it's not resolved soon it will probably finally push me over to Google Music. Thanks for your help!