Premium account, but I still have to sync locally

[flyingkittans]

I have Spotify Premium, and it works fine on my laptop. But whenever I want to listen to songs from my iPhone playlists (songs which are definitely in the catalogue), they're all greyed out unless I've synced everything with my computer. I tried to sync everything, by they way, and sometimes the syncing process just stops for no apparent reason. Spotify will be open, both on laptop and on iPhone, yet my playlists will stop syncing halfway through. That wouldn't be an issue, though, if I could just get the streaming service to work.

tl;dr - Why can't I stream music from my playlists on my iPhone?