Moodle has no quiz import/export fonctionnality.
When you go to Question bank -> Export you only export the questions of a category but not a quiz
So latter when you import this file, you only import the questions. No quiz is created.
You have to totally recreate the quiz from the begining, as when you created it the first time.
Maybe if you want exactly the same quiz in another course or another Moodle server, you should look at the backup/restore functionnality, and not use export/import wich is for questions only ?