From this description I can't determine exactly what the problem may be that is causing the leaves of your windmill palm to fail to lengthen properly. Windmill palm is somewhat susceptible to a disease called lethal yellowing, but that usually affects the oldest leaves first, and gradually moves to the youngest ones. Is it possible that insects, such as mealybugs, are feeding on the palm bud in the main stem and causing the the fronds to remain short? Try digging around a bit in the growing point to see if you find cottony masses. If mealybugs are present, treat them with insecticidal soap, horticultural oil or insecticides containing acephate (Orthene).