Good luck! I think one advise I can give you is to take notice of how you use the British spellings and American spellings, i.e. "s" instead of "z" and other stuffs like that (I'm just guessing though). When I did my IELTS, I also got 6.5 in writing. I'm guessing because I've written it American way. Hahaha..But I wasn't bothered that time coz I only did the General Training as a requirement for the fiancee visa.