Trailer Brake / Brake Controller Question

op wrote:
When I set up the brake controller (following proper procedures and manually applying the trailer brakes at about 25mph) I cannot get the trailer brakes to come close to locking up, even when the brake controller is set to the highest setting.

I've bought probably 10-11 GN flatdeck/enclosed trailers and put them in commercial service.
My current 5th wheel is the fourth rv trailer I've owned. All were new or used units with the same Dexter/Rockwell American trailer axles we use on our rvs.
The first thing I did to all my trailers brakes was do away with all clam shell type connectors and go with a soldered connection. Just this alone allowed me to adjust the controller high enough to slide the tires even at max loads. To those that don't understand the reasons to have that ability of being able to slide the tires ....they probably never will.
If it didn't I went to the brake magnets and ran a ohms check to see if they were at peak operating performance per specs. If they weren't I replaced them with new or uprated ones........ if offered.

We had a poster on one of the trailering forums say his brakes were getting 12 volts but found where a couple of wires were down to 2-3 strands where the factory had pulled them across a sharp corner of the frame.
And of course a bad ground in the system may cause poor braking performance.