Why is the back button (Navigation Page) not showing on iOS?

Hi Guys,

I'm pretty new to Xamarin and I have made an app for my Wedding next year. The app is a cross platform app and is written using Xamarin.Forms. I am using a navigation page to navigate through app like so:

MainPage = new NavigationPage(new NewWeddingApp.MainPage());

However the back button (navigation bar) is not showing on iOS but is on Android.

Any ideas?

Many thanks in advance