Drains lines keep backing up on a new septic system?

We bought this house 2 years ago and from day one the drain lines would back up all over the side walk. They came out and dug up the entire yard and re routed all the drain lines and now its backing up all over the driveway. Its always the last line that that always does it, but it does spout up in areas through out the yard as well. They said they used "good" sand. Now they claim they are out of answers and have no clue what else to do?? They have thumped the ground several times and our yard is raised up a descent amount. Any idea what it might be? They refuse to do anything else and now we might end up in court.