The answer to 'why is my backflow preventer won't flush' is that the most common causes are a clogged or stuck backflow preventer valve. This can happen due to debris, mineral buildup, or a mechanical failure in the device. Symptoms include the backflow preventer not flushing or draining properly, which can lead to water pressure issues or potential backflow contamination.
To troubleshoot, first try manually flushing the backflow preventer by opening the test cocks and drain ports. If that doesn't work, you may need to disassemble and clean the internal components. This is a job best left to a licensed plumber, as improper disassembly or reassembly can compromise the backflow preventer's function. In some cases, the entire unit may need to be replaced if it is worn out or damaged.