I've recently run into an issue with syncing offline playlists:
the sync will work for a few songs, and then get stuck on a certain song.
It looks like it's 'Downloading,' but it can go for hours and nothing will happen. As a result, none of the songs after that one/none of the playlists after that one can sync. If I unsync that playlist and try to sync another, it will work, though it often gets stuck on a different song in the second playlist.
I've tried the usual (restarting app, restarting phone, using WiFi vs. 4G), nothing seems to work. Any ideas?