what medications have you used..Not all dandruff are same you have to try this and that to find which works best for you.
Shampoo containing either Selenium sulfide,Ketoconazole,Salicylic acid and sulfur,Pyrithione zinc,Coal-tar preparations might be helpful.If it doesn't work you might be using the wrong one,consult doctor for a good recommendations.
While using shampoo rub your scalp well and leave it for at least 5 minutes. After 5 mins rinse it thoroughly,if there is residue of shampoo it may aggravate skin problems.So be sure to rense it thoroughly.