With soursops, even with good pollination, it is normal for many flowers to fall off and only a few to produce fruit, but the fruit production improves as the tree gets older. Hand pollination is possible but can be difficult. This article explains how to hand pollinate the related tree Atemoya in the Flowering Behavior section:
Soursop should be the same, but the schedule is: In the early evening, collect pollen from male flowers. Store it in the refrigerator overnight. Apply the pollen to the stigmas of the female flowers early the next morning.