Riona I have experienced that as well. Maybe you didn't plant early enough. I'm zone 9b and I have to start when it's still hot. You may have to start your fall garden in August, maybe even July. It's strange to start cool seasons crops in the hottest time of the year, but next year find a shady spot and experiment with some cool veggies you want to plant. Or pop them in between established summer crops.
The cool thing at least for me is everything will just hang out, and do nothing, but when I get temps the plant likes it will take off, and suddenly produce like crazy. I had a lot of critter problems, so I had to plant my fall/winter crop very late, I'm experiencing exactly what you are talking about now. My peas are about 12" tall, the broccoli is just hanging out. Lettuce, spinach and carrots are growing, but quite slow. It's ok, I don't know about you, but I think fresh peas are worth the wait. I plant a lot, and still hardly any ever make it to the house. We all love to snack on peas right in the garden. Hands down my favorite veggie to eat that I grow. Don't give up, it just takes a bit to figure out the timing, and even then things happen, it's what makes gardening such an interesting challenge. Good luck, and have fun.