The answer to 'why is my sewer line won't flush' is that a clogged or blocked sewer line is the most common cause. This can happen due to a buildup of debris, tree roots growing into the pipe, or a break in the line. Symptoms include slow draining, gurgling sounds, or sewage backing up into your home.
To fix a clogged sewer line, you should first try using a plunger or a drain snake to clear the blockage. If that doesn't work, you may need to call in a professional plumber to inspect the line with a camera and use specialized equipment to clear the clog or repair any damage. Attempting to dig up and replace the sewer line yourself is not recommended unless you have the proper skills and equipment.