Xbee pro s1 distance problem


I am using 4 series 1 xbee module 2.4ghz PRO s1 with digimesh fw version 8070.
They works fine and are connected together in a mesh network that i can discover using the xctu software, but there is a problem, i am not be able to communicate with gateway if the motes are 5 meters away from it, when i perform the range test the RSSI value decrease (-100/-105dbm), i have already tried to increase PL=4, but RSSI value change to -98dbm and some pkt are lost… the xbee modules are all in line of sight so i can’t understand what could be the problem. Any suggestion??

What do you refer to as Line of Site? It may be visually LOS but not RF LOS. How high above all obstructions are the gateway and modules?