Hi girls. I always had dandruff and used a regular sulfate anti-dandruff shampoo. When I started CG, this harsh shampoo was prohibited and I started to co-wash. That's when my nightmares started! I tried many things : non-sulfate shampoo (which dried out my hair and didn't work), 5, 10, 20 drops of tea tree oil in the shampoo and nothing, pure apple cider vinegar (it really burned my skin and didn't word), anti-dandruff-non-sulfate shampoo (helped a little at the begging but didn't work for long time) and finally concentrated tea tree oil with olive oil (didn't work neither).
I was really thinking about quit CG, when I read in internet that people use to confound seborrheic dermatitis and dandruff (if you have it at your eyebrows, for example, its seborrheic dermatitis). And searching a cure for my real problem, I finally found that RAW HONEY could eliminate the problem. There is a serious article about that, you can read it :

Therapeutic and prophylactic effects of crude ... [Eur J Med Res. 2001] - PubMed - NCBI

I started with processed honey but it was less efficient. Basically you put it with 10% warm water in your scalp for 3 hours every 2 days during 4 weeks. And then once a week in order to never have the problem again. I only used 1 hour but I think it is enough for me. If you have already tried everything, give a try, it's worth.