Corn is a heavy feeder, especially of nitrogen. If you are growing in the same area it may be a fertility depletion thing.

you might try planting pole beans or peas with your corn. My grandpa used to plant those together so the peas/beans could climb the corn.

The "Three Sisters" of the Native Americans are corn, beans, & squash. Get the corn started first, then plant a few beans around the stalks. Plant the squash between the rows of corn.

Corn is a heavy feeder. The beans produce nitrogen from the air & help feed the beans. The squash shades the soil.

'Pup

And if you planted the sunflowers between corn stalks you may have affected their ability to cross pollinate which is done by the wind not insects.
Mike

I had read your post and found this information before I saw that someone else had given you the reason. But here it is anyway!!!

Grow corn in a block of at least three rows, as it is wind pollinated. In order to take advantage of full sun, plant rows in an east to west fashion.

Hope that helps you!

Here is a link that might be useful: Where the information came from!