Diagnosing noisy power lines
For power line communication devices to operate correctly they should be on the same leg of the power lines coming into the house. Your house has two power legs and a neutral leg. Voltage from each leg to neutral is nominally 120 volts AC. Voltage from leg 1 to leg 2 is nominally 220 volts. The power line modules work best when all devices are connected to the same leg. If one device is connected to leg 1 and the other device is connected to leg 2, the signal could have to return to the utility transformer (on the pole and could be blocks away) before reaching the other leg.

The quick fix, if you can, is to swap breakers in your main panel so that all power line modules are on the same leg. If you can not do that,this bridge may help. Although this is designed for the "X10" system, they all work pretty much the same and could work as well for your system.

The breakers alternate from leg 1 to leg 2 as shown.