debugger will not stop on breakpoints

Hi. I'm new to Xamarin Forms. I'm using Xamarin 5.7 (build 661). The debugger will not stop on any breakpoints for iOS or android.

Even if I manually add a break like this (no luck):
System.Diagnostics.Debugger.Break();

I've tried 'debug iPhone' and also 'debug iPhoneSimulator'.

Any ideas would be appreciated. I must be missing something simple....

Cheers