Yup, agree, poor pollination. If the air is still when the flowers and flowering, walk through the plants and give them a little shake, you will see the pollen floating around in the breeze - this ensures nice full cobs. My urchins did it for me, and I did it again a few days later, and our cobs are packed to the very tips. Even if you are only growing a few plants, get them close and in a blockor little group, as mentioned.