Tracks visible in Spotify playlist on iPhone but can't make them available off line

[user-removed]

Hi. I am new to Spotify so hopefully someone can help. I have managed to add half a dozen albums to my Spotify account and they are visible on my iPhone as playlists when I open the Spotify app. Trouble is that some albums have a green downward pointing arrow  beside them which I believe means I can play them off line. Other playlists all the tracks have a grey downward pointing arrow and these are not playable off line. Can't figure out why some playlists are available off line and some aren't. Nothing obvious is jumping out. Anuy ideas? Thanks

[David]

The grey arrow means they are due to sync but haven't yet. You may need to connect to a Wi-Fi network for these to continue syncing.

[user-removed]

thanks for your reply. i am at home and my iphone is on wifi now and the spotify app is open. no synching is taking place however. do i need to invoke the synching on my iphone or the desk top client or wouldn't it happene automatically? thanks