I have also been struggling with automatic sync and finally figured it out.
You need "Open iTunes when this device is connected" enabled on each device, which I disabled ages ago as it was annoying. I guess nowadays since you need iTunes running all the time anyway (please Apple please make a daemon for the next version of iTunes, I hate clutter) I may as well leave it ticked.
Since enabling that, it syncs when plugging into power as it should do.
It's certainly not obvious... I mean, iTunes is already open. But I think several versions ago the option said "Automatically sync when this device is connected" and the functionality has remained the same. Indeed I've always had to click "Sync" and just never really thought about it.
All in all you need to:
* Make sure "Automatically sync when this device is connected" is enabled for both devices
* Make sure "Sync this device over Wi-Fi" is enabled for both devices
* Go to iTunes Preferences, Devices tab, make sure "Prevent {iDevices} from syncing automatically" is unticked.
* Make sure iTunes is running at all times.
Then barring any technical stuff like firewalls, it ought to work.