Depending on the variety you have, and whether you planted by seeds or transplants, it might just be that they aren't ready to form the heads yet. Some take more than 10 or 12 weeks.
Another possibility is that the plants are getting too much nitrogen, which encourages leaf growth over flowers (the heads). Depending on the formula of Miracle Gro that you are using, it tends to have higher nitrogen than phosphorus or potassium. Broccoli is a heavy feeder, but it's best to use a balanced organic or slow-release fertilizer if you are applying it every few weeks. Otherwise, too much nitrogen may result in hollow stems and may discourage heading. If you choose a fertilizer high in nitrogen, you might try only applying it about 3 or 4 weeks after transplanting, a smaller amount when the heads are just developing, and then after you harvest the main head (to encourage small side shoots to grow, which you can also harvest). (Before planting, the soil should be amended with composted manure or similar to provide rich nutrients.)
Hot temperatures may also prevent heads to develop, or encourage the plant to bolt (form a flower stalk immediately instead of first developing compact heads).
Depending on the variety you have, and whether you planted by seeds or transplants, it might just be that they aren't ready to form the heads yet. Some take more than 10 or 12 weeks.
Another possibility is that the plants are getting too much nitrogen, which encourages leaf growth over flowers (the heads). Depending on the formula of Miracle Gro that you are using, it tends to have higher nitrogen than phosphorus or potassium. Broccoli is a heavy feeder, but it's best to use a balanced organic or slow-release fertilizer if you are applying it every few weeks. Otherwise, too much nitrogen may result in hollow stems and may discourage heading. If you choose a fertilizer high in nitrogen, you might try only applying it about 3 or 4 weeks after transplanting, a smaller amount when the heads are just developing, and then after you harvest the main head (to encourage small side shoots to grow, which you can also harvest). (Before planting, the soil should be amended with composted manure or similar to provide rich nutrients.)
Hot temperatures may also prevent heads to develop, or encourage the plant to bolt (form a flower stalk immediately instead of first developing compact heads).