Spray starch can just sit on the surface of the fabric (especially if the fabric has not been pre-washed). If you iron the fabric before the spray starch has been absorbed, you get the problems you describe.
Ways to avoid this, as others have mentioned, are to (1) preheat the fabric so the starch absorbs faster, (2) spray the wrong side of the fabric, (3) spray and then wait for the starch to be absorbed before starting to iron, and/or (4) spray and dry the fabric completely before starting to iron (steam will activate the starch). Your ratio of starch and water would work fine for this method.
What I like to do for fabric is mix up my solution of Sta-Flo and water and "paint" it on the fabric with a large wall painting brush, then toss the fabric in the dryer. After it comes out of the dryer, I iron with lots of steam. Can't do this with pieced tops, but it's much faster than spraying for starching yardage.