FlowChart.NET starts creating undo/redo records for modifications done on a diagram after your set

UndoEnabled = True

But then if you immediately call

UndoManager.Undo()

there isn't any record created yet, so nothing is undone. If you want to use the undo/redo functionality, set UndoEnabled somewhere in your application initialization code (e.g. a Form constructor or OnLoad handler), so FlowChart.NET record modifications from the very beginning.