Have a look at this Simple nRF24L01+ Tutorial. I have deliberately kept everything simple to make the examples as reliable as possible.
IMHO it is impossible to know whether the failure is in sending or receiving.
If you have a pair of Unos (or a Mega, or Leonardo) it would eliminate any uncertainty associated with the Atmega8. Then when you know the system works you can try the code on the Atmega8.
...R