You should also verify the brake shoes have been installed properly. The magnet only starts the process. Most of the braking comes from the front shoe being pushed down by the drum. It in turn cams the rear shoe out which does most of the braking. They work much, much better going forwards than backwards. If they are installed wrong their function is greatly reduced.
Do you remember ever backing out of your driveway and then finding your parking brake set when you tried to go forward? That's typical of how drum brakes work.
And they will work on heavy trailers too. Ever see disc brakes on a semi trailer?