Just found this solution which worked for me - the last post by "Mr Tyrell"
https://discussions.apple.com/thread/3395963?start=0&tstart=0
For the lazy, here's the text.
Ok, i finally got automatic sync working. I had the same problems as others - could do a manual wireless sync from the iPhone Settings App, but automatic sync would not start when plugged in and charging. The iPhone showed up in the iTunes sidebar, the "Sync with this iPhone over WiFi" option in the iPhone settings in iTunes was checked, the iPhone and Mac were on the same network (over WiFi - no ethernet connection), and the "Prevent iPods, iPhones and iPads from synching automatically" option was NOT checked, and i still couldn't get automatic syncing to work.
Here's what i did
- made sure the iPhone was showing in the iTunes sidebar
- turned off WiFi on the iPhone, leaving it still showing in the iTunes sidebar
- Ctrl-clicked on the iPhone in the iTunes sidebar and selected "Sync "iPhone"" (the name may be different depending on your device and what you've called it)
- Let iTunes work trying to sync the iPhone it couldn't find until it showed an exclamation mark where the eject symbol is in the sidebar
- hot the eject/exclamation symbol and waited until the iPhone wasn't showing in the sidebar
- Connected the iPhone with a cable and let iTunes re-recognise it
- in the iPhone settings checked "open iTunes when this iPhone is connected" option and hit Apply (NOT Sync)
- Ejected the iPhone in iTunes
- turned the iPhone WiFi back on and let it connect to the WiFi network
- made sure it was showing in the iTunes sidebar
- connected it to my charger
- and wireless syncing started automatically!
Probably not all of this is necessary - I'm not sure which step fixed the problem, but my guess is the "open iTunes when this *** is connected" preference is the important one. Also i hadn't synced my iPhone in a couple of days, so that might be important. My guess is it won't start automatic sync if you've synced it relatively recently.