Hi! I've been having this issue for about a month now and it's so frustrating. 

I have a premium account and iPhone 5c. Over 1 gb of free space. I've tried everything that has been posted before, editing the files name, uninstalling the app in both, my computer (MacBook Air) and my phone, checked my wifi connection, clearing the caché, deleting the devices, etc.

I've created a new playlist with local files and i can play it on my computer, but on my phone they appear greyed out and once I press on the "available offline" button, it just gets stuck in "waiting to download" mode. Nothing else happens. I've reinstalled the app multiple times and the error persists. The only way of seeing the mp3 (although greyed out) is by creating a new playlist. The default Local Files playlist is remains empty. I have updated the app in both my computer and iPhone.

Thankssss!