I have a network of XBee-PRO 900HP S3B modules with DigiMesh 200Kbps firmware ver. 8074. For part of this network, I have a module seated in a tree (u.FL version connected to a dipole antenna) that communicates with a module seated on the side of a railway track (wire antenna) a hundred metres away. For the past few weeks, these radios have been communicating without issue---when I examine the network using XCTU's network working mode, strong or moderate two-way connections are consistently established between these two radios. I have removed and replaced the radios for battery changes, and the connections remained as strong before.
Recently, I removed the modules again to change batteries and replaced the modules in the same positions and orientations as before. However, XCTU's network working mode showed that only one-way connections could be established: The radio on the railway track reported a -83 dBm connection to the radio in the tree, but even after several scans, the radio in the tree reported only "?" for its connection with the radio on the track. Confirming this behaviour, a broadcast transmitted from the tree radio was not received by the track radio.
What does it mean when one radio can "see" another, but that other cannot "see" the first? And why might the connectivity of the radios change, even though I placed them back in the same locations with the same orientations?
Any ideas or suggestions would be appreciated.