You have a wring issue. It is either a bad ground or your not getting the correct voltage at the wheels. You are saying it is drawing 11.3 amps. Where are you checking it??? Also what is the voltage right at the magnet?? That is a better measurement of what is going on. You can have a 11.3 amps at 8 volts and not have much magnet force.
Also what did the back surface of your drums look like??? This is where the magnets run. If that surface is scored they will not work very well. Did you put the spring behind the magnet that pushes in out against the drum??? I forgot on one wheel and the brake would work sometimes and sometimes not. Drove me nuts until I pulled the drum back off.
I have two trailers with Dextra 10K axles. They work good if you keep them maintained. The only issue is the wiring will break inside the insulation from all the bouncing around/wiggling the wires do hanging down to the axle.