nrf24 as TX and nrf51 as RX: ESB

Question

HI, I have a remote control with nRF24L01. It was talking with Gen1(uses nRF24L01) product via ESB. Now, I am developing Gen2 product*using nRF51822xxaa) and I need to support backwards compatibility with the remote control, in other words, I need to make nRF51 to talk with nRF24L01 via ESB. I am able to receive from nRF24L01(this means, address, crc, addr byte length are correct). But, when I send ack from nrf51 to nrf24 with payload, nrf24 is not receiving it. (when I use two nrf51s, one as tx and the other as rx, they both receive and transmit without any problems). Please help. 
 This is the init settings for nRF24L01: 
 hash define RF_CHANNEL 40 //physical channel frequency MHz (2400+RF_CHANNEL)

hash define RF_POWER_DB 3 //0:-18dBm, 1:- 12dBm, 2:-6dBm, 3:0dBm

hash define RF_ADDR_WIDTH 4 //the address is 4 bytes

hash define RF_RETRANSMITS 6 //auto retransmit

hash define RF_RETRANS_DELAY 750 //wait for the ACK (us)

hash define BROADCAST_ADDR_BYTE1 0x7E

hash define BROADCAST_ADDR_BYTE2 0x7E

hash define BROADCAST_ADDR_BYTE3 0x7E

hash define BROADCAST_ADDR_BYTE4 0x6E

 // Enable the radio clock
 RFCKEN = 1;

 //close all pipes
 hal_nrf_close_pipe(HAL_NRF_ALL);
addr[RF_ADDR_WIDTH] = {BROADCAST_ADDR_BYTE1, BROADCAST_ADDR_BYTE2, BROADCAST_ADDR_BYTE3, BROADCAST_ADDR_BYTE4};

 hal_nrf_set_address(HAL_NRF_TX, addr); 

 hal_nrf_set_address(HAL_NRF_PIPE0, addr); 

 // Set rf channel 2430
 hal_nrf_set_rf_channel(RF_CHANNEL);

 // Set the RF_SETUP register
 hal_nrf_set_output_power(RF_POWER_DB); 
 //hal_nrf_set_datarate(HAL_NRF_1MBPS); //1Mbps data rate 
 hal_nrf_set_datarate(HAL_NRF_250KBPS);

 // Set CRC16
 hal_nrf_set_crc_mode(HAL_NRF_CRC_16BIT); 

 // Set address width
 hal_nrf_set_address_width(RF_ADDR_WIDTH); 

 // Set auto retries(ARC) and delay(ARD, us)
 hal_nrf_set_auto_retr(RF_RETRANSMITS, RF_RETRANS_DELAY);

 // Enable payload with ack and Dynamic payload length
 hal_nrf_activate_features(); //this chip does not need activate, so the func is empty
 hal_nrf_enable_ack_payload(true);
 hal_nrf_enable_dynamic_payload(true); //dynamic payload length

 // Enable Pipe0 Dynamic payload length 
 hal_nrf_setup_dynamic_payload(0x01);

 // Enable pipe0 with auto ack
 hal_nrf_open_pipe(HAL_NRF_PIPE0, true);

 // Set rx payload width to the number of bytes expected
 hal_nrf_set_rx_payload_width(HAL_NRF_PIPE0, RF_PAYLOAD_LENGTH);

 // Flush tx and rx fifo
 hal_nrf_flush_rx();
 hal_nrf_flush_tx(); 


 // Configure radio as PTX 
 hal_nrf_set_operation_mode(HAL_NRF_PTX);

 // Power up radio
 hal_nrf_set_power_mode(HAL_NRF_PWR_UP);

 ptx_check_ack.enable = false;


**This is the init settings for nRF51822xxaa:**

 (void)nrf_esb_init(NRF_ESB_MODE_PRX);

nrf_esb_set_crc_length(NRF_ESB_CRC_LENGTH_2_BYTE);

nrf_esb_set_datarate(NRF_ESB_DATARATE_250_KBPS);

nrf_esb_set_retransmit_delay(2750);

nrf_esb_set_base_address_length(NRF_ESB_BASE_ADDRESS_LENGTH_3B);

nrf_esb_set_address_prefix_byte(0, 0x7e);

nrf_esb_set_base_address_0(0x006e7e7e);

 nrf_esb_set_channel(40);

 // Load data into TX queue
 my_tx_payload[0] = 0xEF;
my_tx_payload[1] = 0xEF;
my_tx_payload[2] = 0xEF;
my_tx_payload[3] = 0xEF;
my_tx_payload[4] = 0xEF;
my_tx_payload[5] = 0xEF;
my_tx_payload[6] = 0xEF;
my_tx_payload[7] = 0xEF;
my_tx_payload[8] = 0xEF;
my_tx_payload[9] = 0xEF;
my_tx_payload[10] = 0xEF;
my_tx_payload[11] = 0xEF;


 (void)nrf_esb_add_packet_to_tx_fifo(PIPE_NUMBER, my_tx_payload, 12, NRF_ESB_PACKET_USE_ACK);

 (void)nrf_esb_add_packet_to_tx_fifo(PIPE_NUMBER, my_tx_payload, 12, NRF_ESB_PACKET_USE_ACK);

 (void)nrf_esb_add_packet_to_tx_fifo(PIPE_NUMBER, my_tx_payload,12, NRF_ESB_PACKET_USE_ACK);


 // Enable ESB to start receiving
 (void)nrf_esb_enable();