Problems with AsyncTask class and UDP communication

Question

I am trying to send messages through the UDP communication of an android phone a UDP host, according to the buttons on the screen send a specific message, I have a UDP_Service class where I instantiate the datagramsocket and do the corresponding sending:

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;
import java.net.UnknownHostException;


public class UDP_Service {

  private int server_port;
  DatagramSocket skt;
  private InetAddress local;
  private String msge;
  private String st;
  private int msge_lenght;
  private byte[] byteMsge;
  private byte[] resp;
  private int j;

  public UDP_Service(){
    try {
      skt = new DatagramSocket();
    } catch (SocketException e) {
      e.printStackTrace();
      // st = null;
    }
  }

  public void setIP(String ip){
    try {
      local = InetAddress.getByName(ip);
    } catch (UnknownHostException e) {
      //  st = null;
    }
  }

  public void setPort(int np){
    server_port = np;
  }

  public void setMsge(String msj){
    msge = msj;
    msge_lenght = msge.length();
    byteMsge = msge.getBytes();
    resp = new byte[1024];
  }

  public void Enviar(){
    try {
      skt = new DatagramSocket();
      DatagramPacket pqtEnvio = new DatagramPacket(byteMsge, msge_lenght, 
      local, server_port);
      skt.send(pqtEnvio);
      DatagramPacket pqtResp = new DatagramPacket(resp, resp.length);
      skt.receive(pqtResp);
      st = new String(pqtResp.getData(),0,pqtResp.getLength());
      //skt.close();
    } catch (Exception e) {
      // st = null;
    }
  }

  public void close() {
    skt.close();
  }

  public String getRespuesta(){
    return st;
  }

  public int getCont() {
    return j;
  }

  public void setCont(int x) {
    j = x;
  }
}

The method "Enviar()" I invoke it from an AsyncTask, however it only works once, I open the application from the phone and press any button and effectively the corresponding message is sent, but when I press another button no longer nothing happens. The onPostExecute () method of the AsyncTask is not running,I can not see the corresponding toast.

Here the AsyncTask

  public class MainActivity extends AppCompatActivity {

  String msje[];
  String resp[];
  int port;
  UDP_Service UDP_Serv;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    UDP_Serv = new UDP_Service();
    //UDP_A = new UDP_Async();
    msje = new String[9];
    resp = new String[8];
    }

   public void UDP_Client(String mje, int pt, String ip, int c)
   {
    //UDP_A = new UDP_Async();
    UDP_Serv.setMsge(mje);
    UDP_Serv.setPort(pt);
    UDP_Serv.setIP(ip);
    UDP_Serv.setCont(c);
    new UDP_Async().execute();
    Toast.makeText(MainActivity.this, mje, Toast.LENGTH_SHORT).show();
    }

  public class UDP_Async extends AsyncTask < Void, Void, Boolean > {

  private String st;
  private int j;

  @Override
  protected Boolean doInBackground(Void...params) {
   UDP_Serv.Enviar();
   st = UDP_Serv.getRespuesta();
   j = UDP_Serv.getCont();
   //return st;
   return true;
  }

  @Override
  protected void onPostExecute(Boolean result) {
   //super.onPostExecute(result);
   if (result) {
    if (!st.equals(null)) {
     resp[j] = st;
     actualizarUI(j);
    }
    Toast.makeText(MainActivity.this, "TAREA FINALIZADA!", Toast.LENGTH_SHORT).show();
   }
  }
 }
}