Ask the tree experts

It does not seem to be a fertilization issue rather more of a pollination issue. It seems the trees are having a hard time pollinating. This is normal for Pecan trees. Pecan trees either shed pollen late or early compared to when flowers are receptive thus causing difficulty in pollinating. In a landscape setting you want to plant an early and a late pollinator. If you could go back in time, you would want to plant the above suggestion. Also, plant the trees no more than 300' from native species to help with cross pollination. They are wind pollinated, and can pollinate up to 1/4 mile. Unless you are able to plant more native species in your yard it will be difficult for your trees to pollinate and produce "viable" seeds.