Why do we sometimes fail to answer the most easy mathematical questions in exam scenarios?

Its part of our psychology that while we are preparing for your exams we tend to focus on the complex portion more. And even if the easy portion has a significant weightage in the marks-distribution gird, we leave it to the end.
I am not saying that its a bad strategy, but the dilemma here is that, sometimes when we leave the easier portion for the end, we end up getting an incomplete understanding of it. This leads the eventual marks-loss.
Here is what we should do:

1. Never place low importance on the easier portion. Not matter how easy a topic is, try to go through it atleast once. 
2. Lack of appropriate oversight is another cause of  score-loss. While covering the easier portion, keep your mind open. Try to understand it thoroughly (since you might not have enough time to revise this particular portion).
3. When your teacher is teaching you a portion of your exams which you feel is easy. Attend those classes. From my personal experience I am telling you that such an overlooked portion will take its revenge if you do not understand it appropriately.
4. Leave sufficient time (in the end) for the easy-to-cover questions. One day may not be enough. Place atleast four to five days for such portion. Try to attempt such questions, instead of just reading through your notes.