There are several issues here.
1) Powerline noise: Since LEDs are low voltage DC devices, early LED bulbs used electronic step-down power conversion that often generated powerline noise that interfered with X10 communication. This is usually the cause when an X10 command will turn a light on, but not off. Newer bulbs are either using higher frequency power conversion (smaller lighter cheaper) or have eliminated it entirely by using series strings of LEDs that look like incandescent filaments. We have converted almost all incandescent and CFL bulbs to newer LEDs, most with those string filaments.
2) Signal propagation: Until recently, X10 dimmer wall switches had no neutral connection, and both signal and power had to flow through the incandescent bulb load. That worked fine because the tungsten filament provided a relatively low resistance to the X10 signal. Most LED bulbs - whether they use power conversion or just series strings of LEDs - do not provide that low resistance near the AC waveform zero crossing, where X10 signals are transmitted. Hence, the X10 signal does not have a return path, and the signal never reaches the switch. The fix is to swap in a wall switch with a neutral connection to provide the return path for the X10 signal. Newer X10 dimmer switches intended for CFL or LED bulbs include neutral connections. So do X10 ON/OFF switches intended for fluorescent or inductive loads.
3) Signal attenuation: Some LED bulbs with power conversion circuitry include line filters to block switching noise from reaching the powerline. That line filter can also attenuate X10 signals. The solution is to boost X10 signal levels.
4) Back when we installed landscape lights almost 2 decades ago, I used a XPFM to control the step-down transformer. That is a real "transformer", not a step-down power supply that could be a noise generator. If you are using switching power supplies, you could try installing the X10 XPPF line filter between the XPFM and the power supply to block any noise from reaching the XPFM.