The active ingredient in Head and Shoulders shampoo is zinc pyrithione. If Head and Shoulders shampoo becomes ineffective against dandruff, switching to an anti-dandruff shampoo with a different active ingredient can help.
If Head and Shoulders doesn’t work against dandruff, switching to a shampoo that uses ketoconazole, selenium sulfide, or salicylic acid can help treatment. Your scalp often gets used to the active ingredient so changing the shampoo can provide enough of a switch that your scalp is not resistant to that treatment.