Chlorosis, or the appearance of chlorosis is usually caused by lack of iron or nitrogen. Sometimes a lack of certain micro nutrients can add to the problem. If your hedge isn't responding to the application nitrogen with iron the first thing to do is get an accurate soil test. Iron can be "tied up" by either alkaline OR overly acid soil. If it turns out that nothing is too far out of whack with the pH then there is likely some other problem caused by diseases, pests or some other chemical being used in the environment.