Dexter brake issues have questions!!!!!! help

I have a 2015 PJ gooseneck flatbed with hydraulic tail 12000 lb axles electric. about 50,000 miles on trailer and plug in trailer 7way and trailer brakes do not work. 2015 ram 3500 dually. been pulling this trailer with same truck for 3 years and no issues till recently , truck does not recognize trailer hooked up but if you take a test light to junction box and ground on black wire and probe on white wire then remove then brakes are working ,but after you turn truck off then restart then back to same problem. checked with a different truck same issue. i also pull a 2014 bigtex gn25 and brakes work fine so know it isnt truck. checked junction box no issues, check breakway system and works fine. all lights work fine no problems, cord seems ok. i left to assume it might be a magnet issue but dont understand with four magnets that at least one should complete the circut to make truck recognize that it is hooked up. anyone have any ideas? harness back to axles is good no issues.

Each brake magnet measures 4ohm. 4 brakes means resistance should be around 1 ohm. My chevys with integrated trailer brake won't work trailer brakes if resistance is more than 2.5 ohm. If only one brake magnet is online, or the wiring has flaws, the gm system will say "service trailer brake".

Can you measure trailer circuit between blue and white wire? How many ohms?

Make sure your ground is good through the 7 way plug. Too much rust on the hitch and the coupler can cause issues. I run extra grounds to help with age and wear. On one trailer the bolts that allow you to adjust the coupler up and down had wear and were not staying grounded well. Also Check to see if your brakes are auto adjust or manual. They might need to be tightened.