Why is there a depression around my septic tank cover that is pooling with foul water?

I bought my house 3 months ago and we first noticed the toilet flushed incompletely. Now the washing machine is not draining well and foul smelling water is pooling in a depression around the septic tank cover. We have snaked the pipe from inside the house and there were no clogs there. We are having someone come to pump it this week but I wanted to get some ideas about what could be wrong with it. Any suggestions?