Answer: The symptoms you describe can be caused by poor pollination. Even though your tree is self-fruitful -- that is, doesn't require a second variety for pollination -- it still needs to be pollinated by insecs, expecially honeybees. If the honeybee population is low, or if pesticide sprays have killed them off, then the flowers won't get pollinated. This leads to the underdeveloped fruit that you describe.
First of all, encourage pollinators by planting a variety of flowers, and never spray plants with insecticides when they are in bloom. Some people have good luck with bee attractants, such as "Bee-Scent" spray (which you can get from Gardens Alive, http://www.gardens-alive.com).
Most cherry tree flowers are white, though I don't know about Stella specifically.
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