Certified GKH Gardening Expert
Premature fruit drop is normally the result of poor pollination or an insect problem. If pollination/fertilization is at too great a level for the tree to sustain, all of the set fruit will simply drop. Here is an article that you may find helpful (the info applies to plums as well): https://www.gardeningknowhow.com/edible/fruits/apricots/apricot-tree-not-fruiting.htm
Overbearing is another possibility. Without thinning, many fruit trees will automatically drop a number of seemingly healthy fruits when overcrowded. When the fruit is pea size, begin thinning to prevent fruit drop. This will reduce the energy of the tree so it can put more into the production of fruit. You can also thin the blossoms rather than the fruit.
I would also add some bone meal to add phosphorus to the soil. Sometimes fruit trees will drop their fruit if the phosphorus is too low.