... setting up each ( lamp and appliance ) module with a different letter and number code so none of the seven are the same ...
1. Since you only have 7 modules, I would suggest you start with the SAME letter (House Code) for all of them, and assign each one a different number (Unit Code).
2. You said that some places work, and others do not. Starting with a location that works, test each of the modules in that location, to establish that each module is working, and you can control it. Then, you can start moving them to other locations, and determine which locations are not working (using a "known good" module). Keep the controller in the same place during testing, so you don't introduce two changes at the same time.
Line noise (or a signal sucker) would be my first guess, and the dryer coupler might not be enough to correct the problem.
The links that Brian sent are a great place to start.