Notification service won't unsubscribe

IdeaBlade.EntityModel.Server.NotificationServiceManager:UnregisterUser - User unregistered from _Service Name_

   Hello,
I've started looking with fiddler the traffic I've with my WPF application... I've noticed then when I close the application but don't stop the server (what it normally should be in a real production server) I have the NotificationService that continues to send message even if I've called the  Manager.CancelCallback...

my code is :

Repository :

public void RegisterCallBack(string serviceTypeName, string serviceMethodName, Action<SubscriptionOperation> callback, object userToken, params object[] clientArgs)
   {
       try
       {
           Manager.RegisterCallback(serviceTypeName, serviceMethodName, callback, userToken, clientArgs);
       }
       catch (Exception ex)
       {
           //todo : add logging
       }
   }

   public void CancelCallBack(object userToken)
   {
       try
       {
           Manager.CancelCallback(userToken);
       }
       catch (Exception ex)
       {
           //todo : add logging
       }
   }

(no exception are thrown BTW)

on the BootStrapper : 

     protected override void OnExit(object sender, EventArgs e)
        {
            base.OnExit(sender, e);

            if (repository != null)
            {
                repository.CancelCallBack(UserMessagesViewModel.NOTIFICATION_TOKEN);
            }

        }

In my UserMessageViewModel I subcribe as

private void SubscribeNotification()
    {

        if (isSubscribed)
        {
            repository.CancelCallBack(NOTIFICATION_TOKEN);
        }

        var objArray = new object[] { repository.GetLoggedUser(), FilterAncheVisti };

        repository.RegisterCallBack(NOTIFICATION_SERVICE_PATH, NOTIFICATION_SERVICE_METHOD, UserMessagePushCallback, NOTIFICATION_TOKEN, objArray);

        isSubscribed = true;

    }

What can I do wrong?