Suffiecient number of Honey Bees for pollination.
Honey bees are the chief pollinator of fruit trees.
Planting the wrong variety for your area and insufficient hours of cold temperatures during the dormant period for fruit set. Last winter's warm temperatures are a good example of poor fruiting due to insufficient chilling. Irregular watering is allowing the tree to go dry in between waterings. The biggest mistake people make when planting peaches, plums, apricots and other fruit trees is that they plant their trees too deep in heavy clay soil. Either the bud union is buried or it's just barely visible.