If you are someone who freezes or has a mental blank during an exam or is overly anxious you may find that the heightened anxiety is reducing your ability to recall.
Remember that being as prepared as possible and not leaving your study until the last minute can make a big difference in reducing anxiety. Start your study early and make a plan so that you have time to study what you need to without stress. It is also important to note that the more practice tests you do, the more familiar you are with the test so it will be less intimidating and you will be used to working under time limits.
When you are in the exam if you feel anxious, a good strategy is to close your eyes for a moment and take some long, deep breaths. When you open your eyes do some of the questions you find easy or feel confident about. Meanwhile your subconscious will have a chance to recall the things you are struggling to remember. Another trick is try moving your eyes to different directions, ie looking up and to the left etc. This can also sometimes prompt recall.
You could also seek help from a counsellor for techniques on how to manage stress. You can find lots of strategies for managing stress and anxiety.