Answer:
There could be many reasons for this, but essentially it all comes down to test-taking-skills. While knowing the material is half the battle, you must also know strategies in taking exams, such as:
1. English comprehension
2. Process of elimination
3. Finding answers to questions within other questions
These are just a few of the things that may help you. There are entire online courses on test-taking-strategies, if you think that would help you. Otherwise, my advice is: don't give up.