I moved a few hives into blooming sunflowers, and they were THICK in that sunflower patch. But thats because it was a new spot and the sunflowers were the first thing they discovered.
What you may have, is another nectar source the bees are focusing on.. If they have something that is producing well for them they will not change until that source begins to fail. Your sunflowers may be fantastic, but if the bees found a field of Lavender that started blooming in June, and will continue to bloom until September it will take an act of god to get them to switch to the sunflowers.