Which brake controller are you using?
Voltage output via brake pedal is varied based on how hard you brake.
Voltage output via manual lever allows full voltage
Corrosion, bad connections, wiring scheme, magnet condition, brake adjustment all affect how/what works when controller output voltage is applied (varied/full voltage).
Need to check voltage at truck 7 pin receptacle and then at each brake connection to pin point where your issue is.
Also when was the last time you adjusted the brakes of trailer?
And how did you find that only one brake was working when pushing truck brake pedal......explain your testing :H