Throughput and BER test with nRF24L01+ radio and Teensy microcontroller
@Gerben remarked:   


  You can't send the ACK manually. The module itself sends the ACK
  automatically (if enabled that is), once it receives the package. This
  way the sending module only need to be listening for the ACK for the
  shortest amount of time, reducing power usage. To attach data to the
  ACK package you need the writeAckPayload function. But you have to
  prepare this before any package is received