XBee S1 RF Module: Issue determining distance from RSSI unrelated to RF interference

I’m using a set of XBee series 1 RF modules to determine distance from RSSI. Unfortunately, I see no clear relationship between signal strength and distance, but I know that its NOT related to RF interference (same results in a shielded chamber). So that leaves software/hardware as the cause of the problem

Here’s my setup: two nodes are used, where each node is comprised of an XBee S1 RF Module housed on an XBee Shield, which is connected to an Arduino Uno. The source (fixed node) sends a signal to the receiver (blind node), from which signal strength is measured by the receiver and output to the Arduino at varying distances. The source is powered by a 9V Li-Ion Battery, while the receiver is powered via the USB port on a laptop, from which data is read using Arduino software.

If not RF interference, does anyone know what else with hardware/software could be causing me problems?

I noticed a paper where Xbee Series 2 transceivers were successfully used to determine a clear relationship between signal strength and distance, so I know its possible (though they use a digi dev board instead of an arduino for the blind node).

Thanks in advance for your help,