Problems adding local files to offline playlist

[GCWright]

Hello!

Plan

Premium

Country

UK

Device

Samsung Galaxy Ace 4

Operating System

(Android 4.4.4)

My Question or Issue

I have a downloaded playlist on my phone which I use for offline listening (mostly on my walk to and from work), I keep it downloaded and add/remove tracks to it every few days. The past couple of days I've had problems adding tracks which I have as local files on the desktop version of Spotify.

This morning I added an album to the playlist while connected to wi-fi at home. On first attempt the tracks appeared in the playlist but weren't downloading, so I closed Spotify on my phone and restarted it. The second time I saw the tracks were downloading and eventually the whole album had downloaded. This seemed fine until I left to go to work - when I tried to play a track on my phone I got the 'Unfortunately Spotify has stopped' message. When I opened the app again on my phone the tracks were showing in the playlist but were greyed out and unplayable. I don't understand this at all. Where have the downloaded tracks gone to?