I know this is an old thread, but for the benefit of anyone else, I had a similar problem. I had been using a CM15 to control a light in my LR as kind of a security light -- on/off at various times through the night to make it look like someone was up. Things had been working fine for a long time. Then, I unplugged it for a period of time, several months or maybe a year or so. I decided I wanted to get it running again so I plugged everything back in and it worked erratically. Sometimes the light would turn on but not off; sometimes it wouldn't turn on at all.
I did some research and found the suggestion that line noise can cause such problems and the X10 line filters can fix it. Well, I had gotten a couple new appliances and a new home theatre setup. I decided to start with the XPPF filter. I determined that my home theater system could exceed the 5A limit due to the AVR and UPS, so I thought, aw heck, why not try the air purifier then; that's new. Well, that was the culprit!
A short time later I started having problems again. Over the course of a few days I happened to observe that the light would not turn on or off when the ultrasonic humidifier was running. I ordered another XPPF and voila!; problem solved.
So, there are a couple appliances that can cause problems. If you have similar issues, think about what is new or changed in your home since the time your X10 was working properly. I wouldn't have imagined either of those appliacances would cause a problem, but they did. Either that, or maybe something else--like the new home theater setup introduced more noise, and taming down those two appliances lowered the noise enough that X10 works now. FWIW.....