[Discuss-gnuradio] tunnel.py problem

[usrp n210]

At first we able to run the tunnel.py with setting gr0 and then ping to specific machine.

Now when we increase the distance little bit we are unable to ping the machine however if we use uhd_fft to see the signal we get the 

signal.please provide me some pointer related to this issue means how should I debug this problem as when I place the USRP at same places 

like first experiment , I am still unable to ping the USRP it now always show "Destination Host Unreachable"