Modules Work in Some Locations but not Others
Why do some modules work in some outlets and not in others?
If you are having difficulty controlling a module from a particular location, but can control it from other locations, or if the ability to control a particular module is intermittent, then it is possible that the module in question and your controller are on opposite phases of your house wiring or you are experiencing line noise.
Most houses are fed from a 220 volt service, which is then split into two phases of 110 volts each in the circuit breaker panel ("breaker box"). The X10 signals, which are transmitted onto the house wiring on one phase, will normally travel across to the other phase through the breaker panel, pole transformer, etc.
If, however, the data transmission is not making it from one phase of house wiring to the other, you may couple the signal between them by having a qualified electrician install a phase coupler, AKA a phase bridge. They are available here.
Another solution is to have one transceiver on both phases, e.g. one TM751 set to house code "B" for Basement on a basement outlet and another TM751 set to house code "H" for House on an upstairs outlet, that is on a different phases. Simply use the proper transceiver/house code for a module in a given outlet.
Power conditioners, UPS', and power strips with lightning protection commonly have circuitry (capacitors) that eliminate high-frequency signals that are not standard 50/60 Hertz line voltage as part of their design - otherwise creating "line noise". X10 signals are often degraded on circuits with these devices. You may need to remove power strips (a common, but frequently missed, cause of X10 intermittent failures) or other devices to identify what is causing the noise and degrading the X10 signal.
You may be able to solve the problem simply by moving the controller to another outlet. If the controller is plugged into the same outlet as a TV, its output may be reduced by electrical noise the TV transmits onto your house wiring. Moving the controller away from the interfering device into another outlet may resolve the issue.
If you've located the cause of the line noise, and moving the controller further away does not resolve the issue you can plug the offending appliance into a noise filter, found here.