Late Shipment

I have a problem with the shipment. One of our shipment did not arrive yet after one month. I tried to contact the post office and asked them to check but they only give me some numbers and they said that the receiver should be the one who contact the local post office to check about the shipment. What kind of service is this? I thought it's their problem and they need to solve that problem. We are using their service and we already paid for the shipment cost.