Greetings DJ_Jeff_Fa-Fa,
Thank you for using Apple Support Communities, it's great to have you here.
It's possible that you may be missing music in your library after enabling Sync Library. If that's the case, however, this article goes over the exact steps that you can take to get this resolved: If songs are missing from your music library after you turn on Sync Library - Apple Support
From that article:
First, check these things
* Are all of your devices connected to the internet?
* Do you have an active subscription to Apple Music?
* Are all of your devices signed in with the same Apple ID that you use with your Apple Music subscription?
* Is Sync Library turned on for all of your devices?
If the issue continues to occur, go ahead and follow the rest of the information in that article.
We hope that this information gets everything working again.
Many thanks!