I've got an iPhone 6 with over 50GB of space still free.  Most of my songs have downloaded for offline use, which is great, but there are still a bunch showing with a grey arrow rather than a green one.

I've tried turning my phone off and on again, closing out of the app completely and reopening it, and even changing the quality of music available (which took a whole day to then redownload everything so I really don't want to have to do that again if I can avoid it!)  Sometimes if I close out the app fully and go back into it, the 'available offline progress' bar will come up at the top of my screen, but will then disappear a few seconds later and without it having made any progress towards the download.

Any suggestions as to how I can resolve these annoying grey arrows and actually get *all* of my music available for offline use?  The underground isn't such a fun place without music to drown out the noise of the trains!