Your broccoli is actually producing heads because that's where the flowers come from. "Bolting" is when a vegetable goes quickly to seed because of stress. You may not have noticed heads if they were quite small before bolting. This is called "buttoning." It's caused by stressful conditions such as hot temperatures or dry soil.
You didn't say how early you started your broccoli. We suggest getting plants in as early as possible in the spring and also establishing a late planting in August to have broccoli producing during cooler growing months. Make sure your broccoli is getting at least 1 inch of water per week. Mulch will help retain moisture and keep soil cool in hot weather.
Did you add large amounts of manure or some other high-nitrogen fertilizer to your garden soil? That would account for the strong leaf growth in your broccoli.
Buttoning happens more readily with purchased broccoli transplants, as the plants can be stressed before sale. Because you direct-seeded this year and still had the same problem, temperatures, moisture, or fertilizer could be the cause.
