Apply the brakes with the trucks applicator. Walk around closely with a simple compass if the compass points toward the wheel, that brake magnet is working.
Do it again by pulling the emergency brake cable which will activate all the brakes from your trailer battery.
Trailer drum brakes are notorious for needing adjustment. Our supposedly forward self adjusting Dexter 8k brakes do not do well. Every couple K miles, I got to jack each wheel and manually work the star wheel so the brakes work at maximum.