I did some experimenting with a spare MS14A and RR501 on a HC that is not transceived.
By disconnecting my CM15A antenna and going to the far end of the house, I prevented it from hearing the RF.
The PLC log entries from the RR501 are the same as those when the CM15A does hear the RF, except. of course, when the CM15A sees the RF, there is an additional 'Receive RF entry'.
This means that you cannot use the AHP activity log to determine who is sending the PLC command
Assuming that you did not edit out a Receive RF command, it appears that your RR501 is sending the E15 shown in your log.
The RF signal from the MS13A contains the address and the function in one transmission (HC, UC, and ON) so this is not likely to be a problem
The question then becomes whether the RR501 is not sending the 'ON' following the HC,UC PLC transmission or whether it is sent and lost enroute. Because each address and command is sent twice, it seems unlikely to me that noise is only affecting the function code transmission and not the address, however, it is possible. An XTBM plugged in adjacent to the RR501 could help determine this.
If you have no means of seeing the PLC commands on the power line, I would place the RR501 close to the CM15A (ideally same circuit) and test it.
First, poll it using AHP and confirm that it responds.
Using AHP turn E1 on and off and listen for the relay.
If OK the PLC receive and send is probably OK
Try the MS13A on your E15 looking for the address and ON commands
Change the MS13A unit code and see if that affects anything. Try C11 (remember to change the HC of the RR501 too) as the unit code bits are complementary.
Try another RF transmitter if you have one.
If still missing the 'ON' command, then there is a problem in the RR501 RF processing.
If this all is OK, then you have the most common problem (but usually also the easiest to get around) with X10. Either noise or weak signal is preventing your CM15A from hearing the RR501 transmissions
As you say, it should work. If the RR501 is confirmed to be the problem and you are up to it, I would open it up and check the area of the PCB where the 20V Zener diode is located. This diode gets very hot and is a common failure point. Look for a big capacitor (2.2 μF 250V), a big resistor (22Ω red-red-black) and a 100 μH choke (Blue body - Brown-Black-Brown) and the Zener is located near the side of the PCB just south of the big capacitor.
Sorry for the confusion. I edited my original post to be clearer. The signal in log #548 came from the MS13A. Since my original post I have removed all the house codes from being transceived from the CM15A. The result is the same. No "Receive On" is being transmitted. I've tried the RR501 in different locations in the house being to try several that are on the same phase as the CM15A...no difference. I have filters on any suckers and all my other devices work fine.
I might end up moding my CM15A, but this configuration should work....Argh!