Its not about you are native or not. I have seen many native speakers did v bad in IELTS. Its how you express yourself in the exam.
Every module is created in a way that nobody, I repeat, nobody can cheat his/her way into it.
If you know what you are doing, only then you can score.
Plus, IELTS is very clear on every module and the band they give in it. In the official IELTS website, they have clearly mentioned what you need to do in each module to get good bands.
Every paper is marked on certain charateristics and it happens that we focus on one or two and miss few on the way resulting low score.
I have taken IELTS twice. Never took any training since using English in office for past 10+ year. Here is my experience:
First: I did well in L R & W..not so well in S because it was very tough.
Surprisingly, I got 7 in S and 8.5 in R. I was very sure about W that I will get 8+ but only got 6.5!
Like many I rely on quantity rather quality. I wrote 2 pages letter and 5-6 pages essay, but those examiners at IELTS are not kids.
They know how to rate your paper. Since I got 6.5 in W so it wasn't CLB9 for me. I had to retake it.
Second: I took it very seriously and was very attentive during L. I though I scored 38 and guessed 2. Mind you, it was very tough and questions were very tricky too. However I scored 9 in L.
S went very well..as per my expectations. I was very talkative but to the point. Was cheerful and the examiner was happy with me. I though I easily made 8..perhaps 8.5..turns out I only got 7.5! Surprised here again.
R was way tough than last time. I was in for a big surprise. Not even one section was easy. All full with trick questions. I scored 8.5 last time, I managed only 7.5 second time.
W was big gamble again. The essay and letter all seemed same like last time but I rely on quality rather quantity this time. Used some good vocab, stood on the topic and didn't deviate at all..was precise and focused. I again though I easily scored 8 but was surprised to get only 7.
This time I managed CLB9 with overall score of 8.
Bottom line, IELTS is very tough if you want to score good. Anyone can score 5-6-7. But when you want to differentiate yourself from the rest..7.5-8.5 needs a lot of effort and focus.
It also depends on the test. Sometimes some modules are walk in the park. Sometimes same modules can become nightmares. Prepare for both.
My Final advice:
1. Read the details from the website and see what all they need at each band and see if you are giving them everything to score big.
2. Do practice. Not just L and W..but S and R too. You never know what you might face.
3. Don't take any module lightly. Even if you have scored big in some module last time, it doesn't mean it will be same this time.
All the best!