Q: Why isn't my iPhone6 syncing by wi-fi automatically?

Sync your iPhone, iPad and iPod touch with iTunes using Wi-Fi - Apple Support

The iOS device will sync automatically when the following conditions are true:

• The iOS device is charging.
• iTunes is open on the computer.
• The iOS device and the computer are on the same Wi-Fi network.

If its connected to the Mac it won't automatically wifi sync.

Make sure its lugged into the wall to charge, and is not being used. It should then start to sync. Though only itunes will show it is syncing at the time.