Auto acknowledge failed with nRF 24L01+ and Arduino
To achieve that you have to do the following -

if ( radio.available() ) { // make sure the read will succeed
radio.writeAckPayload( 1, buf, len ); // prep the ack payload
radio.read( &amp;got_time, sizeof(unsigned long) ); // also shunts out ack payload
}


So your program finally becomes - 

#include&lt;SPI.h&gt;
#include&lt;nRF24L01.h&gt;
#include&lt;RF24.h&gt;
int msg[1] = {1};
int rec[1] = {5};
bool stat = true;
RF24 radio(9,10);
const uint64_t pipe[1] = {0xF0F0F0F0E1LL};

void setup()
{
  Serial.begin(57600);
  radio.begin();
  delay(100);
  radio.setAutoAck(true);
  radio.enableAckPayload();
  radio.enableDynamicPayloads();
  radio.stopListening();
  radio.openWritingPipe(pipe[0]);
  radio.setRetries(15,15);
 }
void loop()
{
if(stat)
{
    if(radio.write(msg,sizeof(msg)))
    {
      Serial.print( msg[0] );
      Serial.println("...tx success");
      if(radio.isAckPayloadAvailable())
      {
        radio.read(rec,sizeof(int));
        Serial.print("received ack payload is : ");
        Serial.println(rec[0]);
      }
      else
      {
        stat = false; //doing this completely shuts down the transmitter if an ack payload is not received !!
        Serial.println("status has become false so stop here....");
      }
      msg[0]+=3;;
    if(msg[0]&gt;=100)
    {msg[0]=1;}
    }
 }
}


And the RX part becomes - 

#include&lt;SPI.h&gt;
#include&lt;nRF24L01.h&gt;
#include&lt;RF24.h&gt;
const uint64_t pipe[1]= {0xF0F0F0F0E1LL};
RF24 radio(9,10);
int rec[1] = {2};
int red;
void setup()
{
  Serial.begin(57600);
  radio.begin();
  delay(100);
  radio.setAutoAck(true);
  radio.enableAckPayload();
  radio.enableDynamicPayloads();
  radio.openReadingPipe(1,pipe[0]);
  radio.startListening();
  radio.setRetries(15,15);
}
void loop()
{
  if ( radio.available() ) {
    radio.writeAckPayload( 1, rec, sizeof(int) );
    radio.read( &amp;red,sizeof(red) );rec[0]+=2;
    Serial.print("integer got is : ");
    Serial.println(red);
}
}


Know more about it at the following pages - 
https://shantamraj.wordpress.com/2014/07/19/exploring-the-auto-ack-feature-of-the-transceiver/
https://shantamraj.wordpress.com/2014/11/30/auto-ack-completely-fixed/