Just making sure but .... You do have to have the Mac powered up and iTunes must be running on the Mac in order for this to work.
Make sure that you have set it up correctly. Step 6 is the step that some users forget to do. I made the type bold in the instructions.
- Open iTunes
- To set up Wi-Fi syncing, connect your iOS device to your computer with the included USB cable. Select your device under Devices on the left-hand side.
- In the Summary tab, select "Sync with this [device] over Wi-Fi".
- Whenever the computer and the iOS device are on the same network, the iOS device will appear in iTunes, and you can sync it. The iOS device will sync automatically when all of the following are true:
- The iOS device is plugged in to power
- iTunes is open on the computer
- The iOS device and the computer are on the same Wi-Fi network
- While the iOS device appears in the left-hand column of iTunes, you can select the content tabs and configure sync options.
- Click Apply or Sync to sync the iOS device.
If the iOS device does not appear in the Devices section or you are unable to sync, please see this article for troubleshooting.