Were they newly planted? They can take a year or two to settle and grow roots before they set fruit.

Spot on. Are they planted in the ground or in containers? Fruit trees in pots can suffer from lack of water and the first thing they will do is to drop any fruit. Besides, if they are young trees, then it's best that they don't fruit so the energy goes into stems and roots.

thanks to everyone for the advice and to alextb for the right way to reply