Twisted Box

I am building a set of Corn hole boards (basically bean bag toss) and the boxes are twisted. The overall size is 2’ X 4’ and 4” high. The two by fours make up the frame. The problem I am having is when I set the boards up one corner is 12” off the ground and the other corner is 12.5” of the ground. When I sight down the surface it looks twisted. Everything is straight to start out with and when it is all together it twists. is there a trick to twist it back so it basically is all on the same plane? Any help would be great. I am going nuts trying to figure out how to twist this box. See pics for more clarification.