I recently had issues with my handbrake not working so well, adjustment is tricky...
The adjusters on the cable are for picking up slack as the cable stretches, but once it's streched it won't stretch much more. however, it doesn't make much difference on the handbrake strength, only the engagement position.
It may also be that you need to adjust the engagement position, the service manual does not tell you how to do this, but with the assistance of my mechanic we found that you need to take the slack out of the cable. Also remove the caliper and use a brake caliper piston tool to rotate the caliper's piston outwards on it's thread to make it engage sooner. the thread is how the auto adjustment works when the pad wears. it will take a couple of attempts to get it right and you need to balance the left and right caliper engagement positions using the foot brake. you need two people, one to operate the foot brake, press lightly while your buddy rotates the wheels/tires and see which side engages first and check for minimal drag when pedal is not pressed. once the foot brake engages earlier and evenly on both sides the handbrake should also operate better, it did for me.
while you're there, inspect the pads to make sure the contact area is good. I had some caliper damage due to a broken rear axle and we found that pads on one side were no longer contacting the brake disc evenly which also contributed to the in-effective handbrake operation, this was fixed by some welding on the worn calipers to fix the pads-to-disc alignment, but you probably won't have this issue.
hope this helps.