Assumption is the worst.

I can't seem to get a grasp on necessary and sufficient assumption. It is consistently the worse category for me. I re-read and studied all the sections, paying close attention to assumption, but no change. I tried doing a test untimed, and yet again, assumption was the worst. I can do logic games and reading comprehension just fine, I just always get between 20-30 out of 50 in logical reasoning. What should I do?