Apple Music Library Not Syncing to iPhone from Mac

I have an extensive collection of an artist's unreleased music. I have these files saved to my Mac and a flash drive. Recently I realized multiple songs from multiple albums will not play on my iPhone. These songs do however play on my Mac and my iPad, so I don't think it is a file issue. The songs show up in these albums on my iPhone (I guess meaning the file is on the phone?), however, when tried to play and clicked on, the song will not play and skips to the next playable song in the album. What am I doing wrong? This is the first time in nearly 8 years I have had this issue!