one advantage a V-brake has is the brake cable noodle coming off the side instead of from the center.
This makes it less awkward to route the cable around the seat post.
The key to problem solving is eliminating all variables and getting down to the real problem.
Is the weakness coming from the brake itself or the cable running up to the brake?
Do the cantis move freely on the bosses?
Are the springs giving too much or too little resistance?