Some times it is just little careless mistake in math.
When you get a question wrong, or if you guess, or if you take too long to solve it, do the following:
1. Record what question it was, the question type and what mistake you made (timing, didn't know approach etc.)
2. Read the explanation carefully, multiple times if necessary, until you are confident that you could get the question right if it was asked again
3. Periodically return to your list of missed questions and redo them, making sure you can solve them quickly and accurately
4. See if your missed questions have any pattern (all DS number property questions for instance)
This approach is commonly known as an error log
(search the forums for some good examples).