Your question： Why is 又 used there and not 再 in the above sentence?
First, you need to know the difference of 再 and 又.
1) 再 means“again”，can be used to repeat the same action in the future which haven’t happened yet;
2) 又 means “again” too, but is used for the past to emphasize the event had already happened.
When I finished a cup of beer, I want to order one more, I can say “Waiter, one more, please.” So here I should use “再”。
The teacher had asked me a question, but she spoke very fast and I didn’t get it, here I can say “Teacher, could you please say it again?” The teacher haven’t said it again yet, So here is “再”。
Yesterday I was late, my boss was a little bit angry, and today I was late again, he asked me “Why did you late again?” I was late, this event had happened this morning, so here we should use “又”。
Last time I didn’t pass the exam, and this time I failed again. I got my report and I knew I didn’t pass the exam either. So here we use “又”。
So you need to use "又" in this dialogue.
If you want to know more about how to use "再" and "又"？Please check the free video.