Does not "Automatically sync when this iPhone is connected"

When connecting my 64GB iPhone (6+) to my iMac - ostensibly for charging purposes - it should automatically open iTunes and sync, but it won't.
I can open iTunes myself, and the syncing works (except for some reason the Contacts don't properly update - grrr), but I can't for the life of me get iTunes to open automatically.
I'm on the latest software all round (iTunes 12.2.0.145 / iOS 8.4), have repeatedly checked and unchecked the relevant Options in the iPhone "Summary", but to no avail. My iTunes library is housed on the external drive (due to size).

I've tried pretty much all the suggestions in http://osxdaily.com/2015/03/16/fix-itunes-12-ios-8-sync-issues/ and http://osxdaily.com/2013/06/30/itunes-not-syncing-iphone-ipad-ipod-fix/ bar doing a Factory Reset.

Does anyone have any suggestions as to what the cause and/or fix may be?

Much appreciated... Tony