First, understand it is not necessary that you should get a 7 in writing just because you did so well in the other three modules. Writing is a different kettle of fish and you have to follow the basic principles of good writing to secure your required band in this module.
Did you identify and complete all the required tasks? In both the Writing Tasks?
Did you elaborate the points sufficiently? Did every paragraph lovingly dwell on each point mentioned therein before proceeding to the next one?
Were you always relevant? Or did you write something which may safely be removed without making any difference to your answer?
Did you repeat anywhere? Repetition of words, phrases, points, sentence structures, even sounds maybe?
Were basic grammar rules followed? Did you writing exhibit a fairly wide range of sentence structures – mix of complex, compound and simple sentences?
Were the paragraphs balanced? – all of them roughly of the same length?
Was your response well-presented? Did you give your answer in the introduction with reasons; then elaborate those reasons in the body; and finally restate your answer in the conclusion as given in the introduction?
Was handwriting so bad as to be illegible? If not, it’s not a factor.
If you did all these things, then you should have got a 7. And you should give it for revaluation. Makes no difference – never did – if it is IDP or British Council.
If not, time to go back to the drawing board.