Hi Amy Worth,
If you're working with a course copy/import it's possible that someone may have gotten rid of the question banks that were created when the quizzes were initially setup. For what it's worth, a few weeks ago we got rid of our unfiled questions, altered a quiz question and then boom it showed up in unfiled again. I just tried tonight and that side-effect is no longer working.
Have no fear though, export those quizzes (individually) to create new QTI zip files and then you can import them and create question banks when you do. However, I will caution about importing those quizzes into the same course they came from as you will create a duplicate quiz.

See more at How do I import quizzes from QTI packages?
And in case you do import into the same course and end up with 2 quizzes with the same name: https://community.canvaslms.com/thread/16889-which-quiz-is-the-copy
Hope this advice helps,
Cheers - Shar
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