Often lack of bloom is due to
Often lack of bloom is due to lack of strong sun and heat. The trouble could also be due to a disease or insect. Check for entrance holes, frass, or discoloration at the base of the bud or sunflower head. Are there any other symptoms, such as wilting, or brown areas on the stem? Without a sample, it's hard for us to diagnose. You may wish to visit a local garden center or your cooperative extension office.