Syncing Apple Music across your devices is easier than you think

The main reason Apple Music might not sync your library across devices is that you're using different accounts on each device, so check that first. If you're not sure which account you're using, open the Apple Music app and tap your profile picture on the Home tab. If the email address on the unsynced device doesn't match, you're signed in on a different account. Sign out, then log in using your primary Apple Account.

Now make sure Sync Library is enabled on your devices. On your iPhone or iPad, open Settings and look for Apps. Then, find the Music app in the list and enable Sync Library. On a Mac, open the Music app and click the "Music" menu at the top of the screen, then go to Settings. Now, under the General tab, make sure Sync Library is enabled and click OK to save your settings. The process is pretty much the same in Windows, but you need to click the three-dot button instead to find Settings. If you have an Android phone, this option is enabled by default, and you don't need to change anything. 

No matter the platform, the first sync might take a while, especially if you have a large music library. If you don't see your songs right away after enabling syncing, leave the Music app open and wait a few minutes.