As far as brakes or no brakes, the most important thing is the ground. I had a trailer business for 11 years and the ground was the achilles heal. If you have power and the ground is good, when the brakes is applied by hand, you should be able to hear the magnet engage the side of the drum. How hard it is applied is of course the control. The problem I've had most of the time is the brakes locking up on and empty trailer. So in that case I have a switch for breaking the ground.
If the know the correct power is going to the trailer, and the magnet is not activating, then if everything inside the drum is in place, all that's left to complete the circuit is the ground.
I have a trailer I traded for that was a mess. But with this one I had a different deal. The brakes would not do anything but lock up. The man I traded with didn't even attempt to use the brakes. After crawling under trailer and having a look see,
I found the brake axle to be on backwards. It took me a while but I turned the axle around, rewired everything with a new 7 blade plug and it works like a charm.
Good luck. Has anyone mentioned to check the ground?