The answer to 'pressure regulator won't flush' is that there are a few common causes, including a clogged regulator, a faulty regulator, or a problem with the water supply. Symptoms may include low water pressure, water that won't shut off, or a regulator that doesn't seem to be working properly.
To troubleshoot, start by checking the regulator for any debris or buildup that may be preventing it from functioning correctly. You can try flushing the regulator by shutting off the main water supply, opening a faucet to release pressure, and then unscrewing the regulator to clean it out. If the regulator is still not working, it may need to be replaced. In some cases, the issue could be with the water supply itself, such as low water pressure or a problem with the main shut-off valve.