You may have a quiz which has been set up without random variant questions, ie there is only one question for each numbered question the learner sees in the quiz. This means learners will see the same questions every time they attempt the quiz. 

To make sure learners see a different set of questions each time they attempt the quiz, you will need to set up a question bank with categories and use the add random question option for each question, though you could have a quiz which includes some random variant questions and some questions which always appear each time the learner attempts the quiz. This means writing more quiz questions and devising variants of existing quiz questions.

If the existing quiz has been attempted, you will not be able to add new questions to it or remove questions, it is only possible to make some corrections to quiz text for an already attempted quiz. To update the quiz with substantial changes needed if adding random variant questions, you will need to make a copy of the course and quiz and update the copy, because attempting to update a live course with substantial changes to functionality of assessed activities is not advisable or helpful to learners who may already have completed the original version, especially if the quiz counts towards course completion.