Some corn varieties take up to 120 days from seed to harvest, others are “early” varieties that take only 80-90 days from seed to harvest Possibly, your corn is simply a late season variety that takes a little longer to grow. My friend goes by the adage “ knee high by the forth of July,” and fertilizes accordingly to get good growth by that date. Since corn are heavy feeders, another possibility is that in your case, they need more food and water to really take off. You could try a nice dose of a vegetable fertilizer and water-in well, and see if that makes a difference. I like the Shake and Feed variety of Miracle Grow because it does not burn and corn seems to like it, but your mileage may vary.