The answer to 'water softener won't flush' is that there are several common causes, including a clogged drain line, a malfunctioning valve, or issues with the water pressure. Symptoms may include the softener not going through its regeneration cycle properly or water backing up in the brine tank.
Some potential solutions include checking the drain line for clogs, cleaning the brine tank, and troubleshooting the control valve. However, if you're not comfortable working on the softener or can't identify the root cause, it's best to call a professional plumber to diagnose and repair the issue.