Answer: Fruit set on your plum tree is weather related. Most plums need a minimum of 150 growing days in which to set fruit. I suspect that the years it has produced fruit were mild weather years. Those in which it didn't produce fruit were short summer years. As long as your tree is growing well and is flowering, it should at least begin to set fruit. If the immature fruits drop, or no fruit at all develops, it's probably weather related. There's really not much you can do, except enjoy the fruit on the years your tree can produce it.
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