X10 is not always reliable. Some commands may be missed by devices, there is no guarantee a command arrives and is executed by a device. Noise on the powerline can be a real problem. Luckily you can block the noise coming from devices (like modern switched powersupplies of televisons or computers) with noise filters.
Active Home Vista has nice diagnostic tools on board that can help you see the effect of noise filters, or can help you debug your macros and other X10 programming.: Tray updates, Logging and Statistics.
When you select tray updates you will see the X10 commands a seen by the program (received or sent to the CM11) as a list in a balloon.
When you press the Logging in the menu toolbar a windows pops up that shows you all commands as seen by the CM11.
You can save this log in a file. Pressing logging in the menu toolbar will make the list disappear.
Interpreting the list can be a bit confusing. X10 devices (CM!1, lamp and appliance modules, remote controllers) 'remember' per house code the unit number last used. So you will quite often not see the unit number in the commands in the list.
Via the menu Help - Statistics a windows with useful information is shown:
The Addressed devices tell which house and unit codes have been seen since the program has started.