Q.plum tree only has a few branches blooming
My plum tree is not leafing out this spring like it usually does. There’s only a few branches with buds starting to leaf out.

Though there could be many reasons your tree may be suffering; disease, infestations, pruning issues, soil conditions; I suspect the long cold winter that most of the United States endured may be the cause. Give the tree plenty of time to leaf out.
I would follow fertilizing recommendations in the links below.
https://www.gardeningknowhow.com/edible/fruits/plum/growing-plum-trees.htm
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-pruning.htm
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-leafing-problems.htm