The answer to 'septic tank won't flush' is that there are several common causes, including a clogged septic line, a full septic tank, or a problem with the septic system's drainage field. Symptoms may include slow draining sinks and toilets, foul odors, and sewage backing up into the home.
To troubleshoot, first check the septic tank. If it's full, you'll need to have it pumped by a professional septic service. If the tank is not full, the issue may be a clogged septic line or an issue with the drainage field. Attempting to clear a clogged line yourself could be dangerous, so it's best to call a licensed plumber or septic contractor to inspect and resolve the problem.