The answer to 'why is my shut-off valve won't flush' is that the shut-off valve may be stuck or malfunctioning, preventing the toilet from flushing properly. This is a common issue that can be caused by a few different factors, including mineral buildup, corrosion, or a mechanical problem with the valve itself.
Some common symptoms of a stuck shut-off valve include difficulty turning the valve, water leaking around the valve, or the toilet not flushing completely. In many cases, this can be a DIY fix, but if the valve is severely stuck or damaged, it may require replacement by a professional plumber.