How do I get my App to receive notification even closed or blocked

If I lock the cell's screen and leave the App open, I receive Notifications, but if I close the App I can not receive any more. The problem that when it started working, it was getting in every situation and it suddenly did not work anymore. I'm trying to understand what was done, because out of nowhere there's no way to function. Shipping:

public static async Task<IFCMResponse> SenderMessage()
        {
            FCMClient client = new FCMClient("XXXXXXXXXXX"); //as derived from https://console.firebase.google.com/project/
            var message = new Message()
            {
                To = "XXXXXXXXXX", //topic example /topics/all
                Notification = new AndroidNotification()
                {
                    Body = "Solicitação de Novo Desconto",
                    Title = "Desconto",
                }
            };
            var result = await client.SendMessageAsync(message);
            return result;
        }

Class handling notifications:

[assembly: Permission(Name = "com.global.autorizador.br.permission.C2D_MESSAGE")]
[assembly: UsesPermission(Name = "com.global.autorizador.br.permission.C2D_MESSAGE")]
[assembly: UsesPermission(Name = "com.google.android.c2dm.permission.RECEIVE")]
[assembly: UsesPermission(Name = "android.permission.INTERNET")]
[assembly: UsesPermission(Name = "android.permission.WAKE_LOCK")]
//GET_ACCOUNTS is only needed for android versions 4.0.3 and below
[assembly: UsesPermission(Name = "android.permission.GET_ACCOUNTS")]

namespace Gcm.Client
{
    [BroadcastReceiver(Permission = Gcm.Client.Constants.PERMISSION_GCM_INTENTS)]
    [IntentFilter(new string[] { Gcm.Client.Constants.INTENT_FROM_GCM_MESSAGE }, Categories = new string[] { "com.global.autorizador.br" })]
    [IntentFilter(new string[] { Gcm.Client.Constants.INTENT_FROM_GCM_REGISTRATION_CALLBACK }, Categories = new string[] { "com.global.autorizador.br" })]
    [IntentFilter(new string[] { Gcm.Client.Constants.INTENT_FROM_GCM_LIBRARY_RETRY }, Categories = new string[] { "com.global.autorizador.br" })]
    public class PushHandlerBroadcastReceiver : GcmBroadcastReceiverBase<GcmService>
    {
        public static string[] SENDER_IDS = new string[] {Constants.SENDER_ID };
    }
    [Service]
    public class GcmService : GcmServiceBase
    {
        public static string RegistrationID { get; private set; }

        public GcmService()
            : base(PushHandlerBroadcastReceiver.SENDER_IDS) { }

        protected override void OnRegistered(Context context, string registrationId)
        {
            Log.Verbose("PushHandlerBroadcastReceiver", "GCM Registered: " + registrationId);
            RegistrationID = registrationId;
        }

        protected override void OnMessage(Context context, Intent intent)
        {
            Log.Info("PushHandlerBroadcastReceiver", "GCM Message Received!");

            var msg = new StringBuilder();

            if (intent != null && intent.Extras != null)
            {
                foreach (var key in intent.Extras.KeySet())
                    msg.AppendLine(key + "=" + intent.Extras.Get(key).ToString());
            }

            //Store the message
            var prefs = GetSharedPreferences(context.PackageName, FileCreationMode.Private);
            var edit = prefs.Edit();
            edit.PutString("last_msg", msg.ToString());
            edit.Commit();

            string message = intent.Extras.GetString("message");
            if (!string.IsNullOrEmpty(message))
            {
                createNotification("New todo item!", "Todo item: " + message);
                return;
            }

            string msg2 = intent.Extras.GetString("msg");
            if (!string.IsNullOrEmpty(msg2))
            {
                createNotification("New hub message!", msg2);
                return;
            }

            createNotification("Solicitação de novo desconto", msg.ToString());
        }

        void createNotification(string title, string desc)
        {
            //Create notification
            var notificationManager = GetSystemService(Context.NotificationService) as NotificationManager;

            //Create an intent to show ui
            var uiIntent = new Intent(this, typeof(MainActivity));

            //Use Notification Builder
            NotificationCompat.Builder builder = new NotificationCompat.Builder(this);

            //Create the notification
            //we use the pending intent, passing our ui intent over which will get called
            //when the notification is tapped.
            var notification = builder.SetContentIntent(PendingIntent.GetActivity(this, 0, uiIntent, 0))
                                      .SetSmallIcon(Resource.Drawable.icon)
                                      .SetTicker(title)
                                      .SetContentTitle(title)
                                      .SetContentText(desc)

                    //Set the notification sound
                    .SetSound(RingtoneManager.GetDefaultUri(RingtoneType.Notification))

                    //Auto cancel will remove the notification once the user touches it
                    .SetAutoCancel(true).Build();

            //Show the notification
            notificationManager.Notify(1, notification);
        }

        protected override void OnUnRegistered(Context context, string registrationId)
        {
            Log.Error("PushHandlerBroadcastReceiver", "Unregistered RegisterationId : " + registrationId);
        }

        protected override void OnError(Context context, string errorId)
        {
            Log.Error("PushHandlerBroadcastReceiver", "GCM Error: " + errorId);
        }

    }