I had a similar issue and it turned out to be a broken spring. It can also be a wiring issue. Check the wires going to the back of the brake backing plate on the left side. The splice may have come loose. I also had a rodent problem and rats had chewed through the wires on my coach.
If you have a volt meter, check the voltage at the splice when someone hits the brakes to see if you're getting current flow. If you have current to the splice, you'll need to pull the wheel and drum to see if something mechanical is wrong.