Simon,
Probally not. I wish I could come up with a solid answer for your problem, but I can't. I can send you out a circuit tester and you could test your circuits. This will show you the signal level and allow you to see if certain items are "eating" the X-10 signal. You could return them when you were done. How big is the house you live in. Do you have the X-10 amplified?
Chris