Howdy, look up between the metal duct and the outside wall at the floor joists are they notched and only part of the joist sitting on top of the wall plate? Or is the entire floor joist sitting on the wall top plate( top plate the horizontal 2 by on top of the knee wall studs)? In your photo the arrow points to the back wall as the sloping down direction if this is true remove the sheer wall to reveal the knee wall there too. Check for level- the cement foundation that the knee wall sits on and the the top plate of the knee wall. in both directions from the corner use a 4' or longer level then write back...