C# WPF) DropShadowEffect is sometimes missing when dynamically adding a control
Have you tried to use Canvas to place your rectangles?
Having Canvas to place your shapes such as rectangles, circles, or even graphical paths with random locations is better than placing on fixed layout container such as Grid or StackPanel.
It is quite known that if you just placed shapes with effects such as DropShadow on fixed layout (with arbitrary X,Y locations) such as Grid and StackPanel, your shapes may be clipped. Because the shapes edges will always be checked for pixel boundaries at render time, and often this is caused by overlapped rendering calculations when it tries to recalculate edges of fixed layout controls.
Could be a driver problem with your graphics hardware. Try disabling hardware acceleration and see if it helps.
public partial class App : Application{ protected override void OnStartup(StartupEventArgs e) { RenderOptions.ProcessRenderMode = RenderMode.SoftwareOnly; }}In general I advise against using the DropShadowEffect as it will have a significantly negative impact on rendering performance.