Poor fertilisation. Either you did not plant them in a block or close enough or the wind did not blow the pollen from the tassels on the top to the hairy bits on the cobs below.

Sometimes, when the silks first emerge from the ears, it's a good idea to go through the patch in the morning, and shake the tassels to get good pollination. If you don't have space for planting in a block, try planting them in clumps of 8-12, like the Native Americans do.