Need some advice

I currently have 5 purple haze plants in veg right now. They have been under my 600 watt hps for about 3 weeks now. My question is what should the height of these plants be? I am growing in Canna COCO and my tallest plant is sitting at about 6 inches high. They are all green but they only seem to be getting wider and not taller. Anything I can do to fix this?

Thanks

Raise the light some. 3 weeks is often when they begin to take off in vertical growth.