Someone once called me and said he needed to get an overall score of 7.0 in the IELTS exams. He said that he had taken the exam 4 times but could not get more than 6.0 in his writing.
He proudly claimed that he had taken 18 practice exams, but whenever he took the IELTS exam, he still could not get the score he needed especially in writing.
What he really needed, was feedback. What is feedback you may ask? Feedback is given by a language instructor to a language learner. The purpose of feedback is to highlight strengths and weaknesses.
The problem with the person who called me about getting a higher score in his IELTS writing, was that he never got any feedback. He just kept repeating the same mistakes, over and over again. Even if he took 18 practice exams, it made no difference.
Like any other skill, in terms of the IELTS writing exam, you need someone to highlight where you are going wrong. If you don't, you'll keep making the same mistakes.
Practicing again and again, without feedback results in candidates making the same mistakes again and again.
The moral of the story here, is that no matter how many exams you practice, it is simply not enough. The IELTS exam tests language ability. Improving your language ability is the key to getting better exam results. Feedback from a qualified language instructor could 'fix' that problem you are having, and help you get through that final hurdle.