DidMoveToParentViewController issue

i want to do some work once the navigation back button is pressed so i override DidMoveToParentViewController like so:

public override void DidMoveToParentViewController (UIViewController parent)
{
base.DidMoveToParentViewController (parent);

      if (parent != null)
          return;
      
      //Do some work
  }

Problem is that it work fine on the simulator but not an the actual device ( iPad Mini iOS 8.2)

Any ideas why?