There is definitely some bug. Do a search. Many have reported this.
I had this happen to my wife's iPad. I kept plugging it back in and re-checking "wife sync" and syncing, but when I unplugged, it would disappear from the list.
Today, it just started working. I have no idea why. Maybe because it was out of the house when I rebooted my iMac? I don't know, but it's working again. I guess it healed itself.