The answer to 'pressure regulator won't flush' is that a clogged or malfunctioning pressure regulator is the most common cause. This can happen when sediment, mineral deposits, or debris build up inside the regulator, preventing proper flushing. Symptoms include low water pressure, loud noises from the regulator, or water not draining properly.
In many cases, you can try to fix this issue yourself by shutting off the water supply, removing the regulator, and cleaning or replacing it. However, if the regulator is severely clogged or damaged, it's best to call a professional plumber. They have the tools and expertise to properly diagnose and repair the issue.