You shouldn't have more than one transciever on the same house code as they could both receive the signal transmitted from the same remote (in your case a motion sensor) and then those signals will cancel each other out. You might want to try grouping them in the house by zones (e.g., K for kitchen, E for exterior, G for ground floor, etc.). The other thing I'd suggest is a wireless RF Repeater, which takes the signal from all remotes in your house and repeats them at around 10 times the strength. Significantly increases range.

Here's the one I have. Not too pricey and works great:

[Link: smarthome.com]

This does not have to be set to a specific housecode (the dial is to distinguish one repeater from the next if you have more than one repeater in your house) and will re-transmit any X10 signal it receives, regardless of the house and unit code sent.

Finally, you might want to look for a different motion sensor. I have an Eagle Eye and the range seems to be much more than the 20ft you indicate. They are small and cost around $20 ([Link: smarthome.com])

One other piece of advise, I'd try and avoid using A as best you can; everything from Radio Shack that your neighbors might buy uses A, which results in some falls turn ons or offs. You have so many other choices, maybe try reseting to something different.

Hope this helps