Answer: It sounds like you are having pollination problems. To get fruit, two different cultivars of the same plum family are necessary. For example, two of the same type of tree, or one European-type and one American-type, will not cross-pollinate and the result is often lots of flowers and tiny fruit which drop before maturing. Further complicating plum pollination is the fact that the two types of trees must flower at the same time. Squirrels may be collecting the dropped fruit; I suppose they could also be climbing through the branches and "harvesting"--that would be an interesting sight to see! Late spring frosts may also be damaging your flowers/fruits.
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