Wifi Sync has been a hit-and-miss since iOS 5 for me. Maybe because I'm on Windows.
Maybe the bugs will clear up for you when Yosemite is released.
After upgrading iPhone and iPad, neither would do wi-fi backup/sync. I quit iTunes and restarted. Then I powered off both iPad and iPhone and things are back in working order now.
Wifi sync isn't working for all my iDevices as well, except for my iPhone 5S. I've not powered off the devices, but will do so like you to see if it fixes the issue.
[Update]: Exited iTunes, powering devices on and off then relaunching iTunes solves the issue for now.
Just to update all of you that wifi sync is still functioning after more than 20 hours since I powered off and on all my iDevices.
I hope it lasts!