Pollination problem. Proably a small numbers of plants or planted in single rows. Block planting normally takes care of it.

Each corn kernel has a silk attached to it and must be pollinated for the kernel to develop - some of yours got missed. As stated above, it's best to plant multiple rows in blocks rather than one longer row. Also later ears from a plant are more likely to experience this as there is less pollen still around.

thank-you so much This is definitely the problem. These are my first corn transplants. I had made a mistake with this batch and planted them too far apart. I have more coming that I planted correctly.. It makes sense.. ty