Suspending event not raising using WinRT

I’m having a problem with suspending event on Windows Phone 8.1 using WinRT,
it does not fire. I don’t know why. This is my code:

/// <sumMary>
/// Initializes the singleton application object. This is the first line of authored code
/// executed,and as such is the logical equivalent of main() or WinMain().
/// </sumMary>
public App()
{
    InitializeComponent();

    Suspending += OnSuspending;
#if DEBUG
    this.displayrequest = new Displayrequest();
#endif
}

/// <sumMary>
/// Invoked when application execution is being suspended. Application state is saved
/// without knowing whether the application will be terminated or resumed with the contents
/// of memory still intact.
/// </sumMary>
/// <param name="sender">
/// The source of the suspend request.
/// </param>
/// <param name="e">
/// Details about the suspend request.
/// </param>
private void OnSuspending(object sender,SuspendingEventArgs E)
{
    var deferral = e.SuspendingOperation.GetDeferral();
    deferral.Complete();
}

I set a breakpoint on the line var deferral = e.SuspendingOperation.GetDeferral(); and debugged it with Visual studio. Then
I pressed the start button on my phone and ran another app and waited about 10
seconds. OnSuspending is not running.

Any ideas?