Certified GKH Gardening Expert
Unfortunately, there are quite a few reasons for this. Being in zone 6, you will need to be sure to plant these early enough to reach the 80 to 100 day maturity without exceeding temps between 70 and 80 degrees. High temps are a very common cause.
Next comes fertilization. They need fertile soils, especially nitrogen, in order to produce properly. An all purpose does fine.
Insect problems can lead to lack of sprouts, since it requires quite a bit of the plant's energy to fight.
This is true of infection as well.