What do you mean by "summer broccoli"? Broccoli likes temps in the 40s to low 60s Fahrenheit (approx. 5 to 18 Celsius). Your greenhouse may simply be too warm for the plants to set heads.
But I've harvested broccoli leaves along with chard, kale, and other leafy greens and sautÃƒÂ©ed them all together. Scrumptious!
Sunset Zone 17, USDA Zone 9