I have had the same problem, was about to post a like question (Hope you do not mind my joinging your thread looking for an answer)

My plants (which I purchased seedlings from a nursery) look healthy and a nice color, they just keep growing taller and broader but as of yet no head .

This is my first year trying to grow Broccoli so I have no past experience to judge by. I am looking for the same answer and am curious if it could have been our temps or something/we are both in the same zone (Well some charts say I am 6a others say 6b so I am guess I am best to say 6)

If anyone has any advice, I am trying to learn and go forward!

Well an update is in order.
Do not know if it is the weather or what, but just wait it out some more. Guess time is the secret, I am getting heads forming now, Kind of excited, my lips are watering to harvest straight from the plant, Mmmmmmmmm Good! (I hope)

Broccoli which doesn't head up can be because the plant was stressed in the early days.
Either, too leggy, lack of water or light.