No iPhone calendar event accepted notification?

Hi Everyone,

I have a issue with the 'accepted invite' notification. Basically, when I invite someone to an event they do receive a normal notification, and they can accept/decline etc using the app, but if they accept or decline I do not get an iPhone calendar app notification of their response. I do get a notification on the Mac calendar app. I have checked all notification settings are set to ON on my iPhone. The only calendar I have switched ON is the iCloud account and my iCloud ID is a "@gmail.com" address.

The only way I can tell if someone has accepted my even invitation on my iPhone is if I go to the event itself to check their response.

Why do I not get the accepted invite notification on my iPhone?

Any help would be greatly appreciated.

iPhone 5s running iOS 9.1