my flowering plum tree is not growing leaves
the leaves are small and not filling out
I suspect water deficit and/or nutrient deficiency.
Soil fertility and water management are vital for plant health, foliage and flowering.
Water a wide area not just near the tree trunk, but the entire area of the absorbing root zone which spreads as far as the foliar canopy and more.
Water deeply to wet the soil a minimum of six inches, preferably to 10 - 12 inches. Check this two to four hours after watering, or the next day, by digging down in a couple of spots to see how far the water infiltrated.
Mulch the soil surface to help retain soil moisture.
Fertilize spring and summer with a slow release complete organic fertilizer.