The answer to 'sewer line won't flush' is that a clogged or blocked sewer line is the most common cause. Symptoms include slow draining sinks, gurgling sounds, and sewage backups. Potential solutions include using a plunger, a drain snake, or a hydro-jetting service to clear the blockage.
If the issue persists or the blockage is severe, it's best to call in a professional plumber. They can use specialized equipment to locate and clear the blockage, and may need to excavate the sewer line if the problem is deeper underground.