Guessing ground wire and the run to the trailer via sheetmetal to receiver, to shank, to ball, to trailer coupler, to trailer frame, to axle (via leaf spring mounting hardware), to the backing plate and finally to the magnet
You may have full voltage, but with that path, it will NOT carry much or enough current
If that is the case, then run a dedicated ground wire directly from the controller to the trailer brake magnet wire. Of course some connectors, but chose low impedance connectors.
Also, use a large gauge wire