I had the same problem, did the replacement work myself. In my case that required complete new brake assemblies and drums. The drums were scored and the brake shoes were disintegrating. I don't know what the previous owner did to cause that.
Even after replacement, I still had the drivers side brakes doing all the work, I couldn't even get the passenger side to bed in. That was caused by current imbalance between the brake magnets on the two sides. I modified the wiring to the two sides so that they were independent home runs back to the trailer connector. The way FR wires them the wires go to the drivers side brake first, then over to the passenger side, causing an inherent imbalance. FR also uses insulation displacement connectors which don't have much contact area and are not very reliable. I'd suggest having your dealer run independent home runs and use good quality butt splices with self sealing heat shrink tubing.
Or do it yourself, there is a nice video on it on the etrailer website.