Head & Shoulders Menthol Shampoo Fails to Clear Dandruff, Raises Authenticity Concerns

Unfortunately, I haven't been satisfied at all with the Head & Shoulders menthol shampoo I've been using for a long time. Even though I've been using it regularly for about 5 months, the dandruff problem at my hair roots hasn't decreased at all — if anything, I've started experiencing occasional itching too. Because of this, I've started to suspect the product might not be genuine.
I bought the product from Anpa Gros market, and after seeing no improvement whatsoever following 5 months of regular use, my suspicion that it might be counterfeit has only gotten stronger. So far, I haven't reached out to either the company or the store about this.
I'm requesting that Head & Shoulders investigate the authenticity of the product I purchased and, if deemed necessary, provide a product exchange.