The answer to 'pressure regulator won't flush' is that a clogged or malfunctioning pressure regulator is a common issue that can prevent proper flushing. This may be caused by debris buildup, a faulty diaphragm, or a problem with the regulator's internal components.
Some common symptoms include low water pressure, water pressure that fluctuates, or the regulator not opening or closing properly. In many cases, this can be resolved with a simple cleaning or replacement of the pressure regulator. However, if the issue persists, it's best to call a professional plumber to diagnose and repair the problem.