Spotify Premium - Out of space 12.9Gb limit? still free space on iPhone

[picsoung]

Hi,

I am seen a similar problem but could not find how to solve it. https://community.spotify.com/t5/Help-iOS-iPhone-iPad-iPod-Touch/Spotify-Premium-says-I-m-out-of-spa...

Basically I have a premium Account. 7GB free on my iPhone 5s. But suddently I am no able to sync my playlist offline.
On Spotify Desktop it says that I am using all of 12.9GB  available.

I barely have 600 songs so I have not reached the 3,333 limit.

How could I add more songs ?
thanks