The answer to 'why is my backflow preventer won't flush' is that there are a few common causes, including a clogged or stuck backflow preventer, a problem with the diaphragm or other internal components, or an issue with the water pressure. Backflow preventers are critical safety devices that prevent contaminated water from flowing back into the clean water supply, so it's important to address any issues promptly.
If the backflow preventer is not flushing, the first step is to check for any visible debris or blockages in the device. Try manually opening and closing the valves to see if that clears the issue. If the problem persists, you may need to disassemble the backflow preventer to inspect and clean the internal components. This is a job that is best left to a professional plumber, as improper disassembly can damage the device.