Topic payment, problem when receiving the payment notification

I have this code in C #:

 if (topic.Equals("payment"))
 {
     Hashtable payment_info = mp.getPaymentInfo(Request["id"]);
     var idOrde = ((Hashtable)((Hashtable)payment_info["response"])["collection"])["merchant_order_id"].ToString();
     merchant_order_info = mp.get("/merchant_orders/" + idOrde, true);
 }

With this I get the payment to my account of Mercadopago, but I never get the notification to my server. Does anyone tell me what may be happening?