Gradeitem after duplication

Dear all,

I've written a an activity module for offline quizzes. When duplicating a module instance in a course, my backup and restore code happily creates a new instance. However there is no grade item in the gradebook for the duplicate. I tried it with a normal quiz (mod/quiz) and there is a grade item right after duplication. 
I checked, and the add_instance or update_instance methods don't seem to be called at all (they would introduce a grade item).
Has anyone an idea what I have to change in my code s.t. the grade item appears right after duplication?

My Moodle version is: '2.2.2 (Build: 20120312)';

Thanks and cheers

   Juergen