I had a SignaLinc coupler-repeater that plugged into my dryer outlet. I had nothing but trouble with it. Apparently, it is simply bad design, and simply a isolated defective item. In my 20+ years of X10, it was the only time I flat-out returned an item.
You might want to, however, check into the SignaLinc Booster. I found that works far better. You can place it in the middle of a long run, and it will keep the X10 signals strong.
What you need to figure out, is if the circuits which aren't getting signals, are all on the same phase or not. You could do this by checking out which breakers feed those circuits, and using the diagram found inside the service panel, if they are all the same phase feed. Let us know the results, and possibly we could offer some solutions.