I think there are similar posts to this one, but it doesn't seem to be a problem that is going away, and Spotify seem a long way from rectifying it.
Since about last week, my mobile device ( HTC One X ) seems unable to retain any new playlists that I sync for offline playback. There doesn't seem to be any pattern - they will stay for a few hours, for a day, for a few minutes in some cases. I don't know what the cut off was/is, but my previous playlists are ok. I don't believe I have more tracks that I am allowed ( i have 2128 tracks synced ).
I have tried resetting the cache, reinstalling the app etc etc. But, let's be frank here, I pay for this, so I shouldn't have to bust my balls continuously fighting an app that doesn't do what I am paying it to do.
This is the third major problem I have encountered with Spotify in the past few months. I'm getting rather tired of a service that I used to love, but which now quickly becoming not fit for purpose.