Q: My iPhone 6 plus will not automatically wi fi sync with iTunes 12 when attached to a power supply.

Good day dexv,

There are three conditions that must be met in order for an iPhone to sync automatically over Wi-Fi. It sounds as if you have met one of them, can you check to see if the other two are met as well?

See this article -

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.

Thanks for using Apple Support Communities.

Safe computing,

Brett L 

Hi dexv,

Thank you for using Apple Support Communities. Due to the nature of the issue you described you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action.

Brett L