Here is my "usual suspects" for rf24 radio problems. Some have already been covered.
If you read and, closely, follow Robin2's simple rf24 tutorial you should be able to get them working. That tutorial sure helped me. Run the CheckConnection.ino (look in reply #30) to verify wiring between the radio module and its processor (Arduino).
Make sure the rf24 power supply can provide enough current. I use homemade adapters like these. They are powered by 5V and have a 3.3V regulator on the board.
If using the high powered radios make sure to separate them by a few meters. The may not work too close together.
Reset the radios by cycling power to them after uploading new code. I have found that to help. They do not reset with the Arduino.
Switch to 1MB data rate to catch the not so cloned clones.