pramod on GSM Shield updates only once

This is my code which is copied from " GSM-ParticulateSensor" example

void thingspeakPost() {

 
 
  char itoaBuffer[8];

  char end_c[2];
  end_c[0] = 0x1a;
  end_c[1] = '\0';

// PM25 int to char
  char data25[50];
  String str25;
  str25 = String(Pm25);
  str25.toCharArray(data25, 50);
 
// PM10 int to char  
  char data10[50];
  String str10;
  str10 = String(Pm10);
  str10.toCharArray(data10, 50);

if (inet.connectTCP(thingSpeakAddress, 80)) {

   Serial.println("connected to thingspeak");

    gsm.SimpleWrite("POST /update HTTP/1.1
");
   
    gsm.SimpleWrite("Host: api.thingspeak.com
");
    gsm.SimpleWrite("Connection: close
");
    gsm.SimpleWrite("X-THINGSPEAKAPIKEY:");
 
    gsm.SimpleWrite(writeAPIKey);
    gsm.SimpleWrite("
");
    gsm.SimpleWrite("Content-Type: application/x-www-form-urlencoded
");
    gsm.SimpleWrite("Content-Length: ");
   
   sprintf(sentMsg, "field1=%s", data25);
   itoa(strlen(sentMsg), itoaBuffer, 10);
 
    
    gsm.SimpleWrite(itoaBuffer);
    //inet.httpPOST("api.thingspeak.com", 80, "/update?key=B325M1U9WB2JGC9Z&field1= itoaBuffer");
  //gsm.SimpleWrite("PUT https://api.thingspeak.com/channels/137269/api_key=K9WNHI32J4346GL6");
   // gsm.SimpleWrite(" api_key=K9WNHI32J4346GL6");
   // gsm.SimpleWrite(" name=Updated Channel");
    
     //gsm.SimpleWrite(Pm25);
     gsm.SimpleWrite("

");

     gsm.SimpleWrite(sentMsg);

     gsm.SimpleWrite("

");

     gsm.SimpleWrite(end_c);
   delay(20000);
    Serial.println("update to thingspeak");