First off open the drain clean out plug and let your water flow !
this is where you need to run the snake though but be sure only run until drain is clear ! you do not want to run cable into septic tank ! ! !
Run cold water in kitchen sink while you are running the snake when the water stops
running out on the ground the drain is open, then do a test by running the washer
with a full load of just water to test the drain !