I have 2 Unos with rf24 radio modules attached. I upload your code to them and in the receive serial monitor I get:
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
So the code is not the problem.
Check your wiring. Post a schematic of your wiring. Post a clear photo of your wiring.
How are the radios powered? The number one problem is insufficient current to the radios.
Are they the modules with the external antennas?
I suggest that you read Robin2's simple rf24 tutorial. It helped me to get my radios to work. In the tutorial Robin2 discusses the problem with power and some remedies. I use homemade versions of the rf24 adapters with great success. The tutorial also has a sketch to check the physical wired connection between the rf24 module and its processor. See reply #30.