...I just installed a passive coupler at my breaker box...
...I was told that I don't need a coupler-repeater. I've tried most every other circuit in my house and the signal is not passing to any other circuit....
...is there anything else I can do?...
The only way a coupler might reduce a signal would be if you bridged a signal sucker or noise source over to the other phase. Generally speaking you can't go wrong adding a basic coupler. A repeater couple can cause problems with the repeated signal. A real time boosting coupler on the other hand is a step up from a simple coupler.
Of the 3 main signal problems, your coupler should eliminate the phase one. Next to tackle are the noise makers and signal suckers. Best way to identify is to start unplugging things or flipping breakers off and see if the signal improves. If you find a problem, get an X10 filter for it. Common signal suckers include TVs, surge strips & UPSs, as well as all those battery chargers we have. i.e. Laptop, phone, etc.
While it may not be the most comforting news, you are not the first and certainly won't be the last to have this problem. If you search the forum you'll find many helpful suggestions that are way to numerous for me to sum up in just one post.