What if computer and target iOS devices are on different WiFi networks?

If WiFi is set to ON (on both the Mac and target iPhone/iPad), check to see if the iPhones and/or iPads are on exactly the same wireless network as the MAC. In some homes, there might be a couple of other networks around you, but all devices must be on the same network for the software to be able to perform backups over WiFi.

NOTE: The first backup must be taken through the USB because it can be big in size. The further backups will be incremental and can be taken using the WiFi connection.