Something about your internal ways of functioning is causing you to lock up.
I did a lot of inner work before taking the test, and that inner worked helped a LOT. So, it may make sense for you to some inner work to change what you have going on internally so that you don't experience these issues.
When you practice, what do you do if you don't know how to answer a question? Do you stick with it, even putting it aside for a few hours if you have to, or give up and look at the explanation. If you do the latter, consider sticking with questions for as long as it takes to come up with ways to answer them. Being in the habit of going from stumped to getting a question correct can be super helpful.
Also, since fear and anger can cause energetic blocks, you can work on going through life without ever putting yourself into a state of fear or anger. Work on being Zen and staying in flow at all times. By doing so, you'll become more likely to remain in flow throughout the test.