Music not syncing even though it appears to be

Hello, I have a very annoying problem recently.

I download and import a lot of mp3 files into music, and then sync them to my other devices through the iCloud music library. Recently, tons and tons of mp3s refuse to sync, I select 'Add to Library', and they apparently do, but when I go to my phone they are greyed out and there's the cloud with an exclamation mark in it next to the song. Sometimes it takes several deleting and re-importing and syncing to get a track to sync.

When you go to 'Songs' on the Music app and click on the download symbol icon which supposedly means it was downloaded to the iCloud Library you get this error: "This item was not added to your Cloud Music Library because an error occurred."

I haven't been able to find any explanation. I disabled the sync library and restarted my computer and it didn't fix the problem, I deleted a lot of songs from the library, nothing.

Any ideas? It's seriously aggravating.