Copy/Paste between two windows

I can’t to copy group of nodes from one opened window to another. Both are of the same application. Copy/Paste in same window works well.

Thanks for any insight

Are the models of the two diagrams exactly the same class?
And if so, you haven’t set DiagramModel.DataFormat, have you?

Also, you haven’t set or bound Diagram.AllowClipboard or AllowInsert, have you?

If I think of any other possibilities, I’ll tell you.