Tunnel.py problem

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”