I second the motion on the XTB-IIR. I have the passive coupler and it doesn't do squat. So I splurged for the XTB-iiR and couldn't be happier!
I'll "third" the motion on the XTB-IIR. I've got a +4,000 sqft home on three levels and began my X10 obsession with similar experience to yours: control on one phase, but not on the other. I tried a Smarthome passive dryer coupler plugged in on the third floor laundry room with little positive effect. I then tried the Smarthome active repeater and had a little improvement, but not enough to make me happy. I also tried the XPCR wired-in at the service panel and felt that this didn't do anything at all.
I finally invested in the XTB-IIR. I wired it in at the service panel and relocated my CM15A controler along with the computer server next to the service panel. I did the antenna mod on the CM15A and ran the antenna cable up to a mid-point location in the house. At that point I was getting about a 95% reliable signal response!
After investing a little more time, energy, and the purchase of 1/2 dozen filters, I'm now enjoying 100% effective response througout the house.
The moral of the story: do it right the first time and save yourself a lot of time, frustration, and wasted $$!