The answer to 'why is my pressure regulator won't flush' is that a clogged or malfunctioning pressure regulator is likely the cause. The pressure regulator is responsible for controlling the water pressure entering your home, and if it becomes blocked or stuck, it can prevent the system from flushing properly.
Some common symptoms of a pressure regulator issue include low water pressure, water hammer (loud banging noises), or water not draining properly. To fix this, you may need to clean or replace the pressure regulator. This is a job best handled by a professional plumber, as they have the right tools and expertise to properly diagnose and repair the problem.