What is this "breaker box test" that you speak of?
Basically, there may be something in your house that is causing noise, and/or attenuating (absorbing) the signals.
The fastest way to find it is to check one circuit at a time.
You can do this one of two ways:
1) Turn off one breaker at a time (obviously you'll have to move the modules to a different circuit if you need to turn off the circuit they are on). With that circuit breaker off, test the modules. If nothing changes, then turn that breaker back on, and go to the next one. Keep going through all the breakers, until you find the one that seems to be causing the problem (you'll know it because the system works when that circuit is off).
2) Turn off ALL of the breakers (except the one you are testing your X10 devices on), and test the X10 modules with only that circuit on. If it works, then turn on one breaker at a time, until the system stops working. The circuit that (when turned back on) breaks the system, is the one with the problem.
Once you've identified the problem circuit (and there may be more than one of them), then you need to find which device(s) on the circuit could be the problem. Again, this can be done one of two ways:
1) Unplug EVERYTHING from the problem circuit, and make sure the system is working properly. Slowly plug in one thing at a time, testing the system until it stops working. Pay attention to lights which may have Compact Florescent bulbs, power supplies, cell phone chargers, etc - any of these can cause noise on the system.
2) Unplug things one at a time, until the system starts working. Pay attention to lights which may have Compact Florescent bulbs, power supplies, cell phone chargers, etc - any of these can cause noise on the system.
Once you've identified a problem device (or devices), then you can try to filter it. There are plug-in filters you can buy for most plug-in devices. Lights can be a little harder. Compact Florescent (CFL) bulbs can be big noise sources, so you may need to replace some noisy bulbs with other brands.
Another issue may be cross-phase signals. Most homes in the US are wired with "split-phase" wiring. There are two electrical phases that come from the transformer outside, each bringing in 110 volts. Connecting the two together gives you 220 Volts. Other than the 220V breakers, all of our other breakers are either on one phase or the other. Without a signal bridge or repeater, signals going from one phase to the other need to travel out to the transformer, and then back on the other phase. Most breaker boxes are arranged so pairs of breakers (horizontal rows in a vertical box, or vertical rows in a horizontal box) are on alternating phases. In a vertical breaker box, the two breakers in the first row are on "Phase A," and the next two breakers are on "Phase B," etc. If your modules seem to work only in some places, check to see if the transmitter and receiver(s) are on the same phases or not. you may need a signal bridge/coupler/repeater to pass the signals from one phase to the other.
I hope this helps.