The SS13A's RF is reaching the TM751 as A1 turns it On and Off with its RF signals.
The WS12A's work with their local buttons. So they are probably wired correctly.
Try moving the TM751 to different outlets and branch circuits.
If some locations work and others do not. You have a power line signal problem.
Could be a phase coupling problem as the WS12As maybe on one phase of the homes wiring and the TM751 on the other phase.
That would require a phase couple repeater or finding a location for the TM751 where everything works.
Could also be power line noise or signal suckers.
Try the TM751 in a few locations and see if things change. Like maybe one or both of the WS12As then respond.
http://jvde.us/x10_troubleshooting.htmhttp://www.act-remote.com/PCC/uncle.htm