The blockage or lack of flow could be caused by a number of things.
- Too many bends or sharp bends in the pipe
- Incorrectly set gasketed wax ring can create an obstruction
- Insufficient downward flow of the pipe
- Excessive downward flow of the pipe (rare)
- Insufficient water from the water-saving toilet to get the solids downstream.
Might want to have a plumber run a camera down the pipe before cutting into it, or rent the camera yourself. That might help you from doing unnecessary destructive means to find the problem.