Unable to Sync Music to Offline Playlist?

[Kchak01]

I've had a spotify account for over 8 months now, but I just recently got an iphone, downloaded the spotify app and upgraded to premium. When I turn on the "Available Offline" mode on my "starred" playlist, all the small arrows pointing down under the name of the song are grey, instead of green. I'm not able to listen to any of my music online. Can anyone try to explain to me what's going on. Do I have to plug my iphone into my computer to get it syncing for offline mode.

Anyone's help is much appreciated.