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.
Plum Tree Only Has a Few Branches Blooming
Certified GKH Gardening Expert
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.