Here's the deal.
I've owned many trailers over the years some of them with brakes I could smoke the tires at 70 MPH if I yanked the manual control. Some I could not lock the tires at 10 MPH on ice. (Yes, an exaggeration but not much of one)
I've tried everything known to mankind to fix the ones that had substandard braking and I've learned a few things:
#1. Some trailers had substandard wiring.
#2. Some trailers had substandard brake shoes.
#3. Some trailers had substandard brake magnets.
#4. And the worst of all; some had all of these problems and more!
Sorry I can't give you better news, but this has been my experience.
I would look at each and every system I mentioned above and see what you can find.
Good luck and let us know what you find. :)