With scale insects the mantra is you have to hit them in the crawler stage, before they settle down and make a protective shell. The window for this can be narrow. Maybe try Pennsylvania Cooperative Extension for help with identification and control. There is also a trees forum on this site, the forum your question is currently on is set up to talk about orchard fruits rather than ornamental trees.

I am in California (central coast wine country). It is hot and dry. Zone 8. My soil is heavy clay which I've been told is highly nutritive here. There are ornamental plum trees growing very well all over this entire neighborhood and town. So I know they can do well in this particular clay. My soil holds water well, so somehow I doubt this is a lack of water. Although the nursery told me to water it more. Don't know what to do. Its losing a lot of leaves.

Stanlie, these people are power-crazy and do not want people (G-d forbid) to make a tree or shrub decision by themselves. Only the MANAGEMENT and BOARD know what trees and shrubs we should have. Judging from the extreme overuse of purple ornamental plum trees, trees planted by the curb way too close (resulting in stunting of the shorter species), etc., I have little confidence in the knowledge of the "landscape company" or their "landscape workers". No, they would not allow me to plant a large shrub or small tree. As a matter of face, I recall my neighbor Tom last year telling me quite indignantly that the "landscape workers" removed a Rose of Sharon his wife had planted 5 years earlier, because "it was too close to the building". They didn't even tell him or give him an opportunity to move it. They just snatched it out of the ground, and a rose bush, too.

I don't think they are really happy here. Most of them are always a bit on the annoyed side. If you want to keep it the only thing that comes to mind is more water. How much and how often do you water it? I do see them around the valley a lot, but I can count on one hand how many of them look happy happy. Good luck and keep us posted!