If your block wall faces south or west, the reflected heat may be too intense for sunflowers. Many plants can take the heat, but not reflected heat.

I just wanted to chime in and say how wonderful your veggie bed looks! Thanks for posting the pic, happy gardening!

In my experience, having grown sunflowers for the past 3 years....

The plants starts dying as the flower matures. When the seeds start forming and maturing, the plant dies. I assume then the head will eventually fall and spread its seeds over an area so that it can re-seed in its environment. I have pulled the plant out when the stem is brown and dried. The head is broken off and put near the bird feeders if there are any seeds left on it. The birds that visit my yard start to peck at the seeds.

Not a heat problem. Sunflowers thrive on it.

I have a west facing wall (on the south side of the house) of them that are 7 or so feet high and now at the hottest time of the year they're at their thickest and greenest and are still very soft. They give shelter to the littlest birds who ravage them (and the understory flowers) mercilessly, and they are still thriving. Russian Giants underneath are only a foot tall but they may take over once the first flush has run its course.

I'd go with the age explanation. Plant more; they'll most likely grow until November or later.

Thanks for the feedback and (Grant) for the compliment. I'm going to try planting more. My notes say that I planted the seeds back in March.