Hi Steve, X10 is really not a very suitable lighting control technology if reliable performance is important.

It's based on technology that was developed almost 40 years ago when home electrical systems and the devices plugged into them were very simple.  With all the electronics in homes today, there's just too much that creates interference or attenuates the signals.

Phase couplers and signal boosters will help, but it's nearly impossible to make X10 work 100%.

You'll need X10 test equipment to try and isolate your problem.  I don't work with X10 much anymore, but this is the tester I own... not sure if it's still available: [Link: act-remote.com]

The thing you'll find with X10 is that even if you do manage to get it working, it will can stop working anytime you plug in a new electronic device, or move an electric device to a different outlet.  I've seen entire X10 systems quit working because of a single small night light plugged into an outlet.