I have 2 Unos with rf24 radios attached. I uploaded your code to them and I get
available
Hello World
available
Hello World
available
Hello World
available
Hello World
in the receiver serial monitor. So there is nothing wrong with the code.
How are your radios powered? The number one cause of problems with those radio modules is power.
What modules do you have? The high power modules (with external antenna) can have trouble talking to each other if they are too close together. Try moving them a few meters apart.
CE and CSN must be wired correctly.
I would suggest that you look at Robin2's simple rf24 tutorial. There is a discussion of the power problem and how to solve it in the tutorial. That tutorial is what I used to get my radios to work. Also there is a sketch in reply #30 of the tutorial to test the wired connection between a radio module and its processor that can help with troubleshooting..