Why do only 2 of 4 devices wifi sync with iTunes?

My old 1st Gen iPad (iOS 5), and my iPhone 5 (iOS 8.3) both appear in the device list in iTunes for wifi syncing (not plugged in to iMac)

However, my wife's later version iPad (iOS 8.3) and and her iPhone 6+ (iOS 8.3) do not! I have to update them by plugging them in to the iMac.

Under summary they both have the "wifi sync" box checked under "options". It's curious because they HAVE showed up before in the

device list, but curiously not now. HELP PLEASE)