Offline playlists being reset after I've downloaded them.

[DomeInspector]

I hope this isn't a repost, I didn't really know how to concisely explain my issue so I have no idea where it should go and if there's already a thread on it. I have premium, I recently downloaded all my playlists on my windows phone and then set spotify to offline mode because I have a very low data package at the moment. I come online maybe once every few days to download a song or two I added on my desktop.
In the last few weeks my playlists won't play offline, as if they'd never been downloaded, forcing me to download them again.

Basically I'm being forced to download my songs over and over again to play them offline, I don't have the data to do that. Wondering if anyone can tell me why it's happening so I can fix it.