The answer to 'septic tank won't flush' is that there are several common causes, including a blocked septic line, a full septic tank, or a problem with the septic drain field. The most common symptom is slow or no drainage when flushing toilets or draining sinks and tubs. Solutions can range from simple DIY fixes like unclogging the septic line, to more complex issues that require a professional septic service.
If the septic tank is full, it will need to be pumped by a professional septic service company. If the drain field is clogged or damaged, the entire septic system may need to be repaired or replaced, which is a job best left to the experts. Attempting to DIY complex septic system repairs can be dangerous and may cause further damage.