In addition to what Ingolf is saying, make sure that the 0,0,0 point in the 2D symbol lines up with the 0,0,0 of the 3D symbol. Otherwise, you might place the 3D object in one location, and your 2D symbol will show up in a different location. For example if you have a cube that is 3'-0x3'-0"x3'-0" in 3D and the bottom left point starts at 0,0,0, then make sure the rectangle in 2D (that represents the cube) has it's bottom/left point at 0,0,0.