Tracks/albums that I have downloaded on my iPhone app disappear

[user-removed]

I have a premium Spotify account. I use the iPhone app to download albums and tracks to play offline as playlists. Recently I have noticed that the playlist take a few minutes to load which is frustrating when you just want to open the app and play immediately. Even more recently, the playlists that I have downloaded show as not downloaded, even though I know I have! As a result I have to download again, which is a pain given that I don't have the luxury of unlimited data (in New Zealand). 

I have tried deleting old playlists and downloaded content, but this hasn't worked.

Any tips please?