It almost sounds like in your case. You have a power line signal probelm. The TM751s power line signal can reach the modules in question. While the CM15As power line signal is not reaching the modules in question. The remotes RF signal is enough for AHP to send the signal even though it didn't make it. While the screen does change.
Yep, that's exactly what I was thinking also when I asked about a second transceiver. I think you nailed it.
gluasad, several forum "oldies" have documented that the CM15A has a lower X10 output compared to the older CM11A. I think this also applied to the RR501 and TM751 transceivers.
Have you thought about investing in a repeater and dump the TM751?