I have iphone 6s with 64 gb. I managed to download/offline 75% of my songs, but all the new ones I add to playlists does not get offlined. Even though I have 28gb free space. Most of them go grey ( the logo on the left showing it's waiting to download), but some songs lately also goes red. I have switched wifi, tried downloading over 4g, etc, but nothing works. It's really annoying since I got a phone with bigger space so I could offline more.
Yes, The playlists are marked as green ( meaning all the songs should be offlined) I download in "high" quality . Incase anyone wanted to know this.