No I don't think that's it, because I'm having the same problem on a Nexus 5 and I have over 2gb of free space left. I think this has something to do with the new update, ever since you can't drag and drop music anymore on Desktop PC's ... I don't know what they were thinking when they removed that.
I'm having the same issue. I once had all my local files on my iPhone, but recently I downloaded a lot of new songs that I wanted to import the new songs into my iPhone. Now when I tried to sync over WiFi, it doesn't even sync. Any solutions? Yes I have plenty of memory. I reinstalled the app & desktop (BAD IDEA! LOST EVERYTHING!) & now I am stuck with the limited music provided on Spotify.
I want to listen to my Local Media FIles on my iPhone! SPOTIFY BRING IT BACK!