Local files aren't syncing to iPhone

[user-removed]

I'm trying to add local files to my iPhone.

I've followed all the instructions provided. I added them to a new playlist on the desktop application, synced to my phone correctly (the new playlist appears on my phone) yet when I set 'Available offline' the file doesn't begin downloading.

What gives? I've tried deleting and re-downloading both the PC application as well as the iPhone app, and nothing is working. I've also tried restarting my PC and hard resetting my iPhone. Nothing.

As I have an extensive library on my PC, many of which are not available on Spotify, the ability to sync my local files to my phone in my own playlists was the only reason I went premium. 

Please tell me there is a solution to this.

NB. on a related note I also want to add my local files to a playlist that already exists, so that I can mix my local files with ones saved from Spotify. How do I do this?