Everyone loves a mystery!

I enjoyed reading Sherlock Holmes, solving the puzzle while reading the story has always been a lot of fun. Well I've got a real good mystery but I'm not enjoying it. The symptom is that my home X10 works on one phase of my homes electricity but not on the other. Sounds simple enough, a signal repeater will solve that. I've got a signal repeater and it's working properly. Here's the rub, I have my X10 controller (CM11A) on phase A, I can control my X10 modules on phase B but I can't control my modules on phase A (???). I also have an ESM1 (X10 signal meter) and it shows a 'valid' signal with a signal strength of 4.5V on phase A. To deepen the mystery further I've tried the TI103 (an X10 controller from ACT) in addition to the CM11A and still no change. I've even replaced some of the modules that exist on phase A, still no change. I've got a rather long debugging session ahead of me.