X10 power line signals being sent on the power lines.
Can be absorbed by some electronic devices or corrupted by power line noise around the X10 120KHz power line frequency.
Many times you may have to filter the offending device with an X10 type filter to improve communications.
Most homes have split single phase power. Where there are two incoming power lines and a neutral line. There is 120 volts between either line referenced to neutral and 240 volts between the two lines. For X10 power line signals to get from one incoming line to a device on the other incoming power line. It has to go all the way out to the transformer on the street and back on the other power line. Most times it does not make it or is so weak the X10 module does not respond.
A phase coupler passes the X10 signals between the two power line in the home. Many times connected to the two lines in the breaker box. There are two types.
Passive couples the signals between the two lines at the X10 signal level at the breaker box. The X10 signal will probably be degraded from the wiring from the X10 controller to the breaker box.
Repeater coupler types. Take the received signal on one line. Amplify it back to a decent level and then send it back on the other line.