It's hard to follow exactly what you have; a picture or a drawing might help.
What size is the drain line? It should be 2". If a trap is emptying, it's likely things are not vented properly. Make sure you're using the proper detergent and amount with the WM...the wrong one or too much can be a big problem.
When you moved things, you had to move the trap. Keep in mind that once a pipe becomes a vent is must ALWAYS be a vent, same is true with a waste line...once a waste line, always a waste line - you can't combine the functions in one pipe except in very special circumatances, and then, only in a bathroom group.