What sized light are you using? A 400w HPS will certainly flower 6, but the buds won't be quite as dense as they would if under a 600 or 1k. If you're just using an open wing reflector, it'll be tough to move the light too close to the tops without burning them. But in a well-ventilated air cooled hood or a cool tube, you'll be able to bring the light a good bit closer to the tops, which will result in more light penetration underneath the main colas.