The wizardry involved in getting reliable X10 system is a long tale.

However, I have used the following techniques with some success.

X10RfConfiguration

Install a BoosterLinc or other phase coupler

Use a W800RF32A instead of an MS12A (best $80 you'll spend)

Powerstrips (except the cheapest ones without filtering or suppression) are known to cause problem propagating X10

Computers, monitors, and refrigerators, and especially laptop power supplies may need X10 noise filters.

Take a hard look at whether the workhorse CM11A is causing more problems than it's solving and consider the CM17 instead.