why won't my iPhone 6 notes sync with my macbook air?
How can I get my Notes on iPhone 6 to sync to my macbook air notes?
iPhone 6s, iOS 9.1
If you upgraded your phone to iOS 9 and then also upgraded Notes on the phone to the new Notes features but have not upgraded your Mac to El Capitan (OS X 10.11), that would explain the problem. See the following links:
Before you upgrade your notes - Apple Support
Upgrade your notes with iOS 9 and OS X El Capitan - Apple Support