MulticastSocket.receive not returns even if packet is send

Question

I'm trying to code a service that every 10 seconds sends a message to multicast group. The sender is the following:

public void send(){
    int port = (Integer) config.getValue("MULTICAST_PORT");
    MulticastSocket users = new MulticastSocket(port);
    users.setTimeToLive((Integer) config.getValue("MULTICAST_TTL"));
    users.setLoopbackMode(false);

    InetAddress multicastGroup = InetAddress.getByName((String) config.getValue("MULTICAST_IP"));

    String x = "0";
    DatagramPacket pkt = new DatagramPacket(x.getBytes(), x.getBytes().length, multicastGroup, port);
    while(true) {
        System.out.println("Send Pkt");
        users.send(pkt);

        Thread.sleep(10000);
    }
 }

The receiver have to read the packet and reply.

MulticastSocket multiServer = new MulticastSocket((Integer) config.getValue("MULTICAST_PORT"));
InetAddress multicastGroup = InetAddress.getByName((String) config.getValue("MULTICAST_IP"));
multiServer.joinGroup(multicastGroup);

while(true){
    System.out.println("WAIT KeepAlive Packet");
    server.receive(pkt); 
    DataInputStream in = new DataInputStream(new ByteArrayInputStream(
                    pkt.getData(), pkt.getOffset(), pkt.getLength()
            ));
    if(in.readInt() == 0){ //0 means "OK"
         ObjectSocket skt = new ObjectSocket(InetAddress.getByName(
                                    (String) config.getValue("SERVER_HOSTNAME")),
                                    Integer.parseInt((String) config.getValue("SERVER_PORT")));
          try{
              skt.writeObject(new PacketMessage(new SimpleMessage((String) config.getValue("USER"), (String) config.getValue("OAUTH")), PacketMessage.MessageType.KEEPALIVE));
          } catch (UnregisteredConfigNameException e){
              return;
          }
     }

The output is, for sender:

send Pkt
[10 sec sleep]
send Pkt
(many times...)

for receiver is:

WAIT KeepAlive Packet

Both functions are running on same machine, using two different processes. The MULTICAST_IP is the same (verified). I don't understand why receive() not returns.

MULTICAST_PORT values are different in reference of function executed but this should not be problem. No exceptions are thrown.