Spotify only downloads onto iPhone if song is played

[user-removed]

I have an iPhone 6. Latest OS and Spotify app. 32Gb free space. Have been using successfully for years and suddenly new offline song are not downloading unless I play them, only then do I get the green arrow. Otherwise it hangs on waiting to download grey arrow.
Since I've reinstalled the app (see below) I obviously have no downloaded music cached at all now. So I definitely haven't reached my 3,333 song limit either!

Have spent hours researching fixes and tried everything below:

Logging out
Reinstalling App
Rebooting phone
Logging out of all devices
Removing all synced devices from my account
Removing most of playlists
I haven't got firewall problems.
I've changed 'Auto lock' setting to never and left with app open with wifi and power

But despite this, nothing is happening.

Please help, am at the end of my tether!!!