I've had a quick look through some other posts and may have missed one that is about this so sorry if I have.
Basically my iphone 5 won't play my offline spotify playlist unless I have an active internet connection. I have re-downloaded the songs and the app to see if that would resolve the issue but it didn't. When I open spotify in offline mode it just shows a loading symbol where the songs should be but the songs never load up. As soon as I switch it back to online mode the songs appear and play as if they were always there. I know the songs have downloaded and synchronized as I have a few of them in a collaborative playlist with my partner and I can play them in offline mode. The playlist in question is 1300 songs and therefore well below the limit of 3300 and I have 3gb spare room on my iphone after having downloaded the tracks.
Hi. There's a recent thread on offline issues here. The staff are collating information there to help troubleshoot the problem. The fact that the collaborative playlist works for you in offline mode is a bit strange though so it may be worth trying something else.
On your desktop client, could you create a new empty playlist and copy up to 300 tracks into it then synch the new list to your mobile. Does that new playlist work for you in offline mode?