Your trailer brakes have a manual adjustment using the standard BRAKE TOOL just like your truck brakes do. I jack up my trailer wheel and spin the wheel adjusting the brakes until it start dragging - then back off a 1/4-inch or so...

This manual adjustment is in addition to your electric brake adjustment using the BRAKE CONTROLLER settings..
ETRAILER.COM should have some good info on their site for making the Electric Drum Brake Adjustment..
Found this on-line as well that has some good info with more photos...
Electric Drum Brake AdjustmentRoy Ken