In my opinion, the reason is lack of practice and understanding. The topic regarding directed numbers needs to be conquered fully because if the person doesn't, then the rest of the chapters until form five would be hard. Practice, practice and practice. That's the key. In fact, my teacher told me that at least 2000 questions need to be completed to achieve just a B+ in add maths. However, if there's no interest in Maths, then that would be another problem.
Some of the causes of poor performance in mathematics include: lack of concentration in class, negative attitude, slow learning and lack of commitment to learn the formulas. However, if you realize you are a slow learner or you do not understand, you can request for extra tuition from the teachers in charge.