Just to review to make sure I understood the problem:
1) Using the CM15A as your RF transceiver, the switch works properly ON & OFF when commanded by a wireless remote.
2) You don't have another transceiver that could also be relaying the wireless commands to the powerline.
3) Commands from AHP sent through the CM15A will not turn the switch ON, but they will turn it OFF.
4) This is a solid failure, and not an occasional missed ON command from AHP.
If this is the case, the CM15A is the transmitter for when it works and when it does not. The signal path would be the same, so I don't see how the XPCR could be a factor. That's why I suggested the CM15A might be sending a different ON command when controlled by AHP versus the standard ON command sent by the wireless remote. Of course there might be another factor, such as a noise source or signal sucker interfering with the command when it was sent from AHP.
Jeff