iTunes won't let me drag and drop music to my phone anymore?

Hi there,

Up until recently, I've been able to just drag a music folder directly onto my phone in the "Devices" tab on the right of the iTunes window, and have it immediately sync onto my phone.

But now when I try to do that, it just doesn't do anything at all.

Dragging the album into iTunes first and then trying to sync it from there doesn't help either.

I'm at a loss as to why this would be - as far as I can tell, nothing has changed. Can anyone help me?