When you place a new workplane on an existing 3D face, and you want to reference 3D geometry while you create new 2D geometry, you must change the catch mode to allow you to select geometry from your model. You will need to turn part mode on before selecting. The default 2D catch mode will not allow you to select 3D geometry, so you will need to turn part mode on every time.....Unless, you change the default 2D Catch mode. To do this, select the drop down menu in the catch dialog box that says ""Next Catch"" Select ""Default 2D"" Turn ""Part"" ON. Then, change ""Default 2D"" back to ""Next Catch"". Now, you will be able to catch verticies, edges and centers from the 3D geometry when you are building 2D geometry in workplanes.