So your RF signal is getting out of the room and to your cm15 but the plc signal from the cm15 is not reaching your LM465.
If it wont work in that room only ..then it is a noise,signal sucker problem, or phase issue. The problem doesn't have to be in or near that room. Like others have suggested all you can do is unplug things and see if it starts to help.. Remember other things like:
1. Are there any fluoresce or cfl bulbs in your home. they give off allot of noise.
2. Is this room far away from your electrical box. Long run of wire will cause your signal to be week.
3. check to make sure that room isn't on another electrical phase. Maybe the x10 signal isn't jumping between phase good.
4. LCD TVs , computers ,power strips, phone charger are all bad things all you can do is unplug and see if it helps.