Robin2's simple rf24 tutorial helped me to get my radios to work. I had less luck with other tutorials.
The tutorial covers the problems with supplying enough current to the modules. Failure to supply sufficient current is one of the most often problems. I use homemade versions of this adapter on my radios and have no problems with power,
If you are using the higher powered radios (external antennae) they may not communicate if they are too close together. Move them a few meters apart.
The radios do not reset with the Arduino when uploading code. Cycle power to the radios to reset them. after uploading new code to the Arduino.