HC12 sending back data to another HC12/ping-pong communication style

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1

Hello everybody, I am having trouble to send data back from HC12 receiver to HC12 sender.
I want to have communication like so : HC12 sender sends data --> received by receiver --> add a char --> send it back to the sender to be read.

This is my sender code :

void loop() {
  String str = Raspi.readStringUntil('

and receiver :

void loop() {
  String input = HC12.readStringUntil('}');   //read data from sender 
  char ab = input.charAt(input.length() - 1);
  if (input.length() > 15) {
    if (ab == ']') {
      input = input + "}" +"&";
      //Serial.println(input);
      HC12.print("%");
      HC12.println(input);
    }
  HC12.flush();
}

i've checked data sent from sender to receiver via serial monitor, and it's sent. But I can't send the added string data back to sender from receiver.
What could be wrong? Oh, and anyway, I wish i could do back and forth communication with no using serial monitor.

Thanks before!);  //read from raspi
  if (str.length() > 15) {
    HC12.println(str);
  }
  HC12.flush();

delay(500);
  if (HC12.read()=='%'){                            //read data from receiver
    String str1 = HC12.readStringUntil('&');  //read from receiver until &
    if (str1.length() > 1) {
      Serial.println(str1);
    }
  }


and receiver :

§DISCOURSE_HOISTED_CODE_1§


i've checked data sent from sender to receiver via serial monitor, and it's sent. But I can't send the added string data back to sender from receiver. 
What could be wrong? Oh, and anyway, I wish i could do back and forth communication with no using serial monitor.

Thanks before!