Hi there,
I have the same problem, since I installed Mavericks F7, F8 and F9 are not doing anything (with iTunes). I checked the function key settings and it should be okay, the box 'Use all F1, F2, etc. keys as standard function keys' is unticked. All the other function keys, like brightness, mute and volume setting work perfectly, it's only the iTunes control keys that don't function. What's even stranger is that when I tick the aforementioned box and try to use the iTunes control keys by holding down fn, they still don't work. Please help, it is really annoying having to switch back to iTunes all the time to control the music. (I'm using iTunes 11.1.3 (8).)