nRF24L01 serial like synchronous bi-directional communication

Hello,
I've tried this sketch on 2 Arduino with NRF but I lose most of my packets.
If I try to send 20 times the same packet, I receive it in the best case 1 time in 80% of the tests and 0 time in the other 20%.
I never receive all of my packets.
I don't understand why ...