iPhone 6 won't sync with iTunes

I have an iPhone 6 128GB and I am trying to sync my music from iTunes (on my macbook pro).

I have the latest software updates on both (iOS 9.1 and El Capitan).

I used to have an iPhone 4s which synced perfectly every time. My iPhone 6 has never synced, even when I got it 4 months ago. I have trawled the forums and tried everything:

- I haven't joined Apple Music, so I don't have an iCloud Library to disable

- iTunes Match isn't enabled

- Genius isn't enabled

- I've put it in aeroplane mode and synced again

- There is no music on the phone for me to delete

- My music plays fine in iTunes (i.e. it's not because iTunes can't find the music)

- iTunes detects my iPhone - I can even see the songs on my device I'm trying to sync, just greyed out

- When I try to sync, it goes through all the steps, but just doesn't end up putting my music on my iphone

I'm exhausted of trying to download music onto my new phone which I spent a chunk of money on specifically so I could download a lot of music to it. What's with all the problems, apple?