One other thought:
When you are moving the CM15A to the other systems, you are plugging it into different outlets, and possible different circuits in the house.
Have you confirmed that you can send powerline commands from those PCs through the CM15A at each of those locations?
Perhaps it IS working, but the signals aren't getting through due to a long signal path and/or other noise.
We weren't always testing by controlling a physical device.
The initial problem was on one computer where the recvplc command wasn't seen after a recvrf, and the device didn't switch (it did when changing direct from the CM15).
Taking it downstairs (definately a different ring), I WAS seeing the recvplc (but couldn't see the physical device to check).
A quick test on another machine on the downstairs ring, 2 sockets away, DIDN'T see the recvplc.
I assumed (yeah, I know) that that meant it was the CM15 only transceiving when on my PC.
Does it explicitly show a recvplc after it's sent a command or is that just concidence of the wiring?
When I get access to the PC with AHP on it, I will try and see what the CM15 is actually set to.