My iTunes won't sync my music onto my phone?

Usually when I plug my iPhone into my laptop it automatically and adds any new songs I've put onto iTunes, onto my phone, but it won't do it this time. Every time I try to sync my music onto my phone it goes through the normal phase of backing it up and so on, but when it gets to 'waiting for changes to be made' it stays on that for about 2 minutes and then just goes off and my music doesn't appear on my phone. I've tried dragging the files from iTunes onto my phone, but that doesn't work either. Does anyone know why it's doing this, and how to stop it? It would be really appreciated, thankyou!