Perhaps you are dealing with a voltage loss in the trailer's braking system. In addition to using cheap electric brakes, the wiring is often undersized resulting it voltage loss. Or you might have some grounding issues and sub-standard wire connectors.

I suggest you trace down the brake wires, ground connections and wire connections.

There have been reports of the brake wire being routed through the tubular axle from one side to the other. The wire isn't secured and rubs to the point where the insulation is compromised causing voltage loss to the metal of the axle.

And upgrade to a direct wire from the A-frame to each brake. Use a stranded conductor rather than solid core wire. Also use the next size larger wire 12awg vs 14awg. Encase the wire in a plastic conduit that is securely zip-tied to frame to prevent snagging of flexing. Make a good clean connection at the brakes using a premium connector and insulate it if possible. This modification assures maximum voltage to the brakes, protects the wire from damage and gives you a better brake system.