Lack of pollination due to rainy weather with few bees is often the cause. Do you live in area with wet spring weather? You can encourage more bees with flowering plants in the spring or by adding mason bee houses to attract native bees. Your tree looks healthy for such a young tree. Another reason for a poor harvest is the age of the tree. This tree is still young so give it a few more years before making like George Washington and giving it the ax! Keep growing, Marianne Binetti