Liquid waste overflows, strong unpleasant smell and clogged drains are some of the signs of a failing septic system.
The most common reason for a malfunctioning septic system is a lack of maintenance. For instance, if your septic tank hasn’t been pumped out for many years, it could likely be full of sludge which can clog up the pipes.
See here for more signs that a septic system is not working properly:
How to Solve the Biggest Problems with Septic Tanks and Systems