This can happen if you have an incomplete quiz or if you have taken all the questions in that specific category.
Check to see if you have an incomplete quiz by clicking on Your Quizzes on the QBank page. You will see a quiz at the top of the list with a Resume button to the right. Finish that quiz to take a new one.
If you have gone through all the questions in one category, you will have to switch to a different category to create a new quiz.