The first factor is whether the second question is more difficult than the first one. Is the second time in class generally lower than the first time? If so, it's not your problem alone.

The second factor, if not the above factors, is your problem. Your foundation is not very solid, and you found your weak link in the second exam. Compare the second time with the first time, find out the difference between the two times and make a breakthrough. Then you will have almost no weak links in the future exams.