Manual Sync to iPhone doesn't work with some music
So I'm accustomed to just dragging and dropping music from my library into my iPhone XR via iTunes but as of right now I can only do it with some music. And I can't for the life of me figure out why. I literally dragged one album into the phone, then the next one would not work.
As far as I can tell the only difference is these are songs I just added to library today. In other words, they are on an external drive and I'm just adding them to iTunes now, then trying to transfer to the phone.
any thoughts on what's going on and how to fix it?
iMac 27", macOS 10.15