I think I'm going to give up on fall broccoli. My spring broccoli seems to produce all year long. It seems that next year I should just double my spring planting and forget about the fall crop. That's certainly easier that way.
I find that some of my plants produce big heads in early summer, then produce side shoots for a while. But others take so long to grow that they don't make heads until late summer. After the first head is cut, they still produce shoots for quite a while.
I have three varieties this year: Green Sprouting Calabrese, Blue Wind and Marathon. I'm not sure which is which. (Labeling is not my strength.) I bought Blue Wind because its supposed to produce a nice fall crop. The Green Sprouting Calabrese is the one that gives nice small shoots for a long time. Marathon makes beautiful big spring heads in my parents garden, but I think it makes smaller heads all season for me.
I did plant some fall broccoli seeds in early August. Its still very tiny. Same as last year. Probably I'm just not giving it ideal conditions and it grows slowly.