Kevin,

All homes have noise on the power line. The noise level can vary significantly from one curcuit to another. The X10 signal will almost also find a path from one phase to the other but there's often significant attinuation so the signal on the "other" phase is signifianctly lower than on the original phase. Put this all together and you get situations where a signal can overpower the noise on one phase, or one circuit, but not others. Also, all transmitters are not created equal. Some transmit stronger signals than others and without a test meter you have no way to verify either the noise level or signal strength.