auto acknowledge failed with nRF 24L01+ and arduino

my transceivers are running fine. i 've written a few code and got them running.then i decided to check the auto acknowledgement feature wherein we can return data back to the sender WITHOUT manually changing the radio modes on both units.but i find that the serial monitor always shows "failed to transmit".why is this happening cause otherwise they work fine.

i am using maniacbug's RF24master library.

the Tx code is-

#include<SPI.h>
#include<nRF24L01.h>
#include<RF24.h>

int msg[1] = {1};
int rec[1] = {0};
bool stat = true;
RF24 radio(9,10);
const uint64_t pipe[1] = {0xF0F0F0F0E1LL};

void setup()
{
  Serial.begin(57600);
  radio.begin();
  delay(1000);
  radio.setAutoAck(true);
  radio.enableAckPayload();
  radio.openWritingPipe(pipe[0]);
  radio.setRetries(15,15);
 }
void loop()
{
  if(stat)
  {
    radio.stopListening();
    bool ok = radio.write(msg,sizeof(msg));
    delay(100);
    if(ok)
    {Serial.println("transmitted successfully !!");}
    else
    {Serial.println("faileed to transmit");}
    msg[1]++;
    if(msg[1]>=200)
    {msg[1]=1;}
    stat = false;
    //delay(20);
  }
  if(!stat)
  {
    if(radio.isAckPayloadAvailable())
    {
      radio.read(rec,sizeof(rec));
      Serial.print("received ack payload is : ");
      Serial.println(rec[1]);
      stat = true;    
    }
  }

}

and the Rx part is -

#include<SPI.h>
#include<nRF24L01.h>
#include<RF24.h>
const uint64_t pipe = 0xF0F0F0F0E1LL;
RF24 radio(9,10);
int rec[1] = {0};
int ack[1] = {1};
void setup()
{
  Serial.begin(57600);
  radio.begin();
  delay(100);
  radio.setAutoAck(true);
  radio.enableAckPayload();
  radio.openReadingPipe(1,pipe);
  radio.startListening();
  radio.setRetries(15,15);
}
void loop()
{
  if(radio.available())
  {
    Serial.println("payload available....");
    bool ok = radio.read(rec,sizeof(rec));
    if(ok)
    {
        Serial.println("message received !!");
        Serial.print("integer got is : ");
        Serial.println(rec[1]);
        radio.writeAckPayload(1,ack,sizeof(ack));
    }
    else
    {Serial.println("failed to receive the message");}
  }
}