Mathematics is notorious for exams like that. But if you look at the reasons why people fail to get a perfect grade at undergraduate level, it's almost always due to honest mistakes or because they didn't learn what they were supposed to learn.

In my experience, studying mathematics is a bit weird. If you are ready to learn a topic, it's probably the field where you can get top grades with the least effort. But if you can't learn something with reasonable effort, hard work is unlikely to help. Doing something else and trying again after a few months might help.