Your sunflowers must have flowered very early as the flower heads on mine haven't even developed yet. If they haven't actually flowered, I doubt whether they will develop seeds, which normally appear and ripen as the petals drop off. If they have flowered, then you will need to let the seeds develop of their own accord, which may take a time yet. Possibly the lack of rain may mean that seeds won't necessarily develop. I suspect the weather has been having a funny effect on lots of plants in terms of their development.