Lift each tire (1 at a time with a jack placed under the spring perch on the axle). While the tire is in the air, spin it then you or a helper can hit the controller override lever to engage the trailer brakes. This will show you if you have one or more brakes that are not functioning. This will most likely not show a 'weak' brake, but will show one that isn't working at all.
Now that you know which brake(s) is not functioning, you can go more in depth to find out the reason why (adjustment issue, faulty wiring, bad magnet, contamination, ect).
2005 2500 Cummins/48RE/3.73, QCLB, 4wd, BigHorn, Edge Juice w/ CTS + Turbo Timer,Transgo Shift Kit ISSPro Oil and LP pressure gauges, GDP 20/2 filters, Custom Diesel Steering Box Brace
'10 Forest River Shockwave Toy Hauler 21'
Honda EU3000I Genny