X10 RF has multiple design issues which result in very poor RF range for things like motion sensors, door/window switches and handheld keychain remotes.

The remotes, motion sensors and door/window switches have LC components for setting the carrier frequency. These tend to be poorly tuned and this can worsen as the LC components age resulting in a carrier frequency that may differ significantly from that of the receiver (e,g. TM751, RR501, CM15A). Second, most have very inefficient PCB trace antennas resulting in very low transmitted power. For stationary transmitters, adding a passive radiator adjacent to the device, as shown here, can result in a large increase in signal strength.

Reception problems are made worse by the design of the CM19A which uses a short length of wire as an antenna shared by both transmitter and receiver sections. Since it is powered via USB, the CM19A antenna is isolated from safety issues related to mains voltage. See the CM19A schematic.

Adding this eggbeater antenna atop the CM19A and connecting it to the receiver circuits, as shown below, should give more than adequate range.

Below, an eggbeater is shown mounted atop an MR26A which used the same case as the CM19A.