The answer to 'water softener won't flush' is that the most common causes are a clogged drain line, a problem with the control valve, or a malfunctioning timer. Symptoms include the softener not going through its regeneration cycle properly, water not draining from the brine tank, or the tank overflowing.
Some DIY troubleshooting steps include checking the drain line for clogs, cleaning the control valve, and resetting the timer. However, if the problem persists or you're unsure of the cause, it's best to call a professional plumber to diagnose and repair the issue. Attempting complex repairs on your own can lead to further damage, so it's generally recommended to have a licensed plumber service the water softener.