As long as your controller says you have a connection, the first thing to do, while parked is jack up one wheel at a time, spin it and have DW hit the brakes. If it barely brakes, but you here it, your magnet is working, but you need an adjustment. Turn the star nut located on the backer plate with a flat head screw driver until it improves. After your satisfied, take the wheel off and drum, inspect the drum for grease or scoring as you may need the drum turned and also inspect the brake shoe. Last inspect and repack your bearings. Put it back together and before letting it back down, spin the wheel again to make sure its working properly then repeat 3 more times on the other wheels. If you need shoe, spring or magnet replacement, rednecktrailersupply.com sells the entire set up, already assembeled. All you do is remove the 4 bolts holding the whole assembly onto the axle, then bolt on the new one, attach two wires, then spin and check for adjustment. Its all very easy to do and on a quiet Saturday you can have all brakes up to 100% and have you bearings packed as well in time for a beverage and game or hammock in the afternoon. Its really not hard to do.
