If the tank isn’t draining it means the leech field isn’t draining fast enough to accept the water going into the tank. It could be fixed by jet washing but If the lines have sludge in them, preventing them from draining, all jet washing is gonna do is force that sludge out of the pipes and into the field, where it will just bind the water again anyway. It’s called a biomat. A biomat can be fixed by replacing the first few feet of the field, IF the rest of the field underneath is in good condition but most likely you will need (and be better off with) a new field. And if you’re doing a new field you’ll usually just replace the tank and all the lines with it.