Sunflower Problem

There are two possible causes. One is a rust fungus infection which is common on sunflowers. It is a particular problem when the weather is hot and humid or the plants receive overhead watering. Remove the affected leaves and be careful to water only the soil.
The other possibility is a small, piercing insect. There are a number of tiny moths which pierce the leaf of sunflowers and lay eggs. If that is the case you will soon see tiny tracks as the small larvae eat through portions of the leaf. These insects are hard to control and rarley cause more than cosmetic damage.
They have no flowered because they are too small. Once there is enough plant, meaning they have grown large enough to support flowering, they will do so.