itunes syncs songs to iphone but playlists don't contain them

I've got itunes 12.7 on a Mac and an iphone 7. I have a couple of playlists that are large (> 1000 songs) that I set up on the computer and sync to the iphone. I recently added a new album, and added some of the songs to each playlist. I have iTunes set up to sync those playlists to the phone. The songs synced, but they don't appear in the playlists on the phone. What's going on?