Are you using the SC1200 as the transceiver for all your remotes to the power lines? If it is the common module used for all the remote tests it maybe the one with the issues.
Can you control any modules or are all not working?
Can you control the modules with the SC1200's Keypad Buttons?
Can you control a module plugged into the same outlet as the SC1200?
Did some tests.
AM466, Radio Shack 49-1000 {SC1200 OEM from X10}, KR32A, SH624 Security Remote.
Console sends an On and Off to the House Code-Light Code and House Code-Light Code+1 setting.
AM466 set to House Code and Light Code set in the console.
AM466 flashed On and Off for Exit Delay, Panic and Alarm. From Console, KR32A and SH624.
AM466 Turned On and Off with the SH624 Light On Off button and Manual Button on the lower part.
AM466 Did not turn On and Off with the KR32A Lights On Lights Off Buttons.
AM466 Did turn On and Off with the KR32A if set to the House Code and Light Code+1 Unit Code.
Signal tests showed the KR32A sent the HC,LC+1 On and Off.
I would also look for noise makers or signal suckers as mentioned earlier.
I have some files with the captured X10 messages on the power lines if you have any questions.