My plum tree - 4 years old but pruned incorrectly in 2nd year so was hoping for fruit this year - blossomed beautifully and tiny green fruits appeared. Now a few weeks later I can't see any fruit on the tree. It's a victoria plum, planted in an allotment. A couple of plots away there is also a plum tree.
What insecticide / fungicide schedule are you using ? You will need at least 3 sprays after dormant oil , if you expect much fruit. I have had surprising success with peaches with no spray ( the squirrels and coons loved them ). However, I had two varieties of plum trees about 50 ft away and I have not seen any fruit larger than 1/2 " diameter in 20 years ( no spray). Apparently some insect larva eat the fruit as it starts to grow. In another location , I grew many types of tree fruit using sprays with good success. Red currants and raspberries will produce well with no sprays. Also hazel nuts do well without spray.