Heidlberg
Did you plant the Broccoli from seed? or did you buy small plants at the store?
The one time my Broccoli behaved like yours is doing I had purchased some small, sad-looking plants and there was a delay in planing (okay, I was lazy and forgot), by the time I planted them they grew just enough to make heads, tiny little heads. The plants were under stress and were tying to save the species.
The next year I bought plants and put them in the ground immediately. Success! I added lime to the soil and lots of compost and had a steady supply of water, never letting the plants feel stressed.
Broccoli is my favorite food in the world.