If you are struggling to improve your score for writing task 2, the solution might be to spend more time planning before you start writing.
I asked the students on my most recent one-day course to spend 10 minutes planning a question about whether the only purpose of prisons is to punish people. Here are the ideas that my students had:
Paragraph 2: prisons do punish offenders
Punishment makes offenders think, reflect, feel sorry, understand the consequences (that we have choices about the actions we take), suffering, revenge / justice for victims, debt to society, taking away freedom and privileges, miss family, deter people from committing crimes in future - fear of prison
Paragraph 3: other purposes of prisons
Reflection time, rehabilitation - help the prisoner to prepare for normal life: new skills, education, courses, qualifications, treatment - reform, reeducate, find employment, re-enter society - address the root causes of crime
With a plan like this, you should find it easier to write a great essay.