Drum brakes have a break in period. I remember this from my old days working at the local gas station. My research on trailer drums lead to an adjustment of pulling the loaded trailer at 25mph on concrete or asphalt, and then hit the manual slider and seeing if the trailer brakes lock. Adjust the max power until just shy of locking up. For my trailer that is 6.0 out of 10. I do that every time heading out. If I had to set the controller to 10 and it would not lock up, I'd say its time for new magnets, which means probably time for new shoes, which means just buying the package backing plate already loaded for installation.