axel12p: The only problem I found, and is shown in the screenshot above, is that every module has to be at least one time in transmission mode before it can listen to the other. So, if I power an Arduino through external power the two modules will never start to communicate with each other. The same happens if for a reason there is a power-off to one of them.
Maybe I'll have to change the code so that the module enters the transmission mode for a while, when it powers up, and then return and remain to receive mode.
I had the same problem. I fixed it by adding the following two lines right after the first instance of: // // Start listening // radio.startListening(); <----FIND THIS LINE, somewhere around line 109
radio.openWritingPipe(pipes); <---ADD THESE TWO LINES AFTERWARDS radio.openReadingPipe(1,pipes);