Hello guys

Bike is a Trek with Tektro Novela mechanical disc brakes.
My rear disc brake doesn’t fully stop the wheel. I’ve adjusted the barrel adjuster and cable tension so that the brake lever has about 2 inches of travel before it engages, but even when I pull the lever all the way, the rear wheel doesn’t lock up. The front brake works fine and stops completely.
I’ve already tried adjusting the cable tension and playing with the barrel adjusters.
I also noticed that at the caliper clamp bolt doesn’t seem to be much cable left sticking out after it’s secured. Could that be the issue, or is something else likely causing the weak braking?