From within a particular project task if attempted to 'Copy Partial Project' of another sub-task, using any name, it does not throw error but nothing is copied.
The Project Tasks might not be referring to Main Project as top_task.
The project tasks and sub-projects should always have top_task set to the main parent project or top task. If they point to a task just a level above them as Top_task and not the main project as top_task then copy partial project fails. It does not throw error but nothing gets copied and a new Task is not created.