Do you have a large central chimney mass? If so, a lot of times the weight of these tends to settle the middle of the house. A lot of times the floor is mounted to it and it goes down with it. You might try to disconnect the floor from the chimney, jack it, and re-support it. A pretty big job, get some help from someone who has done this kind of work before. This can be dangerous if you don't know what you are doing.