The K&B master exam is harder than the real exam. I found some of the question in the bonus 3rd master exam a little ambiguous so I also scored less on it. If you are scoring low on any particular topic, you should study it again before trying any more mock exams. ExamLab is free so there is no harm in trying it. Try Inquisition too...
Take it as Learning Tool after you give the practice exam.find where are weak in those Question.and give the practice exam again..with more and more practice after each exam.
Your concepts will get clear and you will score good marks in the real exam