Easy test would be to jack up each wheel and set the breakaway. If the wheel locks, it has brakes, of not, then tear it down and find out why.
In simple terms, there is the power 12v to the magnet, and from the magnet a ground wire. The front axle may be single homed, meaning one wire powering just that axle, and the two trailing axles split wired, where one wire splits off to both axles.
See below.
If no braking on the trailing axles, then get a trouble light, or multimeter, and tap each wire to see if 12V is present when the breakaway is engaged. Do this at each wheel. If 12V is present, tear down the hub and find out why, if no 12V then back trace or run new wires.