Problem with messagingCenter

Hello,
I've a problem using messages to drive the navigation from an async call.
I try to explain better. My problem is really simple: I've a login page. In this page I subscribe to navigate when a certain message is received:

MessagingCenter.Subscribe<ViewModelBase,string>(this, HelperClass.LOGIN_SUCCESSFULL, (sender, username) =>
            {
                ServiceLocator.Current.GetInstance<INavigationService>().NavigateTo(ViewModelLocator.CouponsPageKey, username);
            });

Then, in its viewmodel, when a user press the login button, a relaycommand is triggered to make an async call to a WCF, in this way:

 public RelayCommand LoginCommand
        {
            get
            {
                return _loginCommand
                       ?? (_loginCommand = new RelayCommand(
                           () =>
                           {
                               Loading = true;
                               HelperClass.ClientWS.LoginAsync(Username, Password);
                           }));
            }
        }

When this service has replied the handler is triggered

    void ClientWS_LoginCompleted(object sender, ValidazioneUtenteCompletedEventArgs e)
    {
        var _res = e.Result;

                if (_res)
                {
                    MessagingCenter.Send<ViewModelBase, string>(this, HelperClass.LOGIN_SUCCESSFULL, Username);
        }
    }

I can see that the message is correctly received because with the debugger the navigateTo is executed.

What's the strange?
Is that in this way I can't see my app navigate to the new page, while if inside of my LoginCommand instead to call the

HelperClass.ClientWS.LoginAsync(Username, Password);

I directly send the message the navigation occurs correctly.
What's wrong?

Thank you