By the time they reach part 3 of the speaking test, many IELTS candidates just want to finish and go home. They've used up all of their energy doing the other parts of the test, and they're happy to have finished the 2-minute talk in part 2. As a result, not many people do really well in part 3.
Here's my tip: Save one last effort for part 3. It's your best chance to leave a good impression on the examiner and perhaps push him / her to give you that extra half band that you need.