Arduino and RF24 library: communication stops after short amount of time
Your question does not provide much information to go on.  Setting up as given in the tutorial is a good start, but does not guarantee that there will be no problems.  It can still either be a problem due to hardware or firmware.  

Firmware problems:


The radio data may be getting corrupted after some time, which shows up as a failed radio transaction
You inadvertently change the register settings of the radio


Hardware problems:


There is noise interference on the radio channel you are using due to the urban environment (wifi, etc).  Does the system work any better on a different channel?
The radio drains too much power and browns out, then will revert to default settings on the IC rather than the ones set by your Arduino sketch.