As a no longer suffering user of X10 now enjoying ZWave, the issue is usually crossing the out of phase 110 legs (every other full size breaker in the panel is on a different leg).

To cross the phases, the signal has to go to the power pole where the phases originate. X10 has a repeater you can install and sometimes people just install a passive one (a capacitor if I recall.)

In a smaller house, signalling to the panel and back out to circuits on the same leg is more likely to work than crossing the phases.