Plum trees fail to bear fruit for both simple and complex reasons. Sometimes the failure to bear fruit is inexplicable. Also, producing fruit sporadically can be common, with many years of fruitlessness followed by a lush crop. Poor weather during the pollination window, a cold weather snap, too much or too little watering or rain, over-fertilization or lack of bees to pollinate can all be reasons. Maturity can also be the issue as plums rarely produce fruit before five or six years of age. Consider and correct cultivation issues where you can, and if all else fails, attempt to shock the plum intro reproductive mode by light root pruning.
Don't use herbicides and insecticides on your plum trees when bees may be present, as this can kill the very pollinators you need to bring the tree into fruit.
Modulate watering to achieve a good moisture balance where the soil an inch or two down feels lightly to moist at times but not consistently wet. When seasonal rainfall is heavy, scale back on irrigation to avoid over-watering.
Shock treat your plum tree in the summer or early fall by driving a sharp spade or garden edger blade down into the soil to sever some root material. Place the cut just beyond the drip line of the tree and make one to three cuts. This is thought to shock the tree into fruiting as the tree senses damage and a threat to its survival.
Refrain from applying fertilizer to your plum tree and if you do so use only low-nitrogen formulations. High-nitrogen fertilizer can discourage bloom and impair fruit development.