You did this to yourself — in the most understandable way possible, and the shampoo is not broken. Dandruff (and its bigger sibling, seborrheic dermatitis) is a chronic, relapsing condition: it's driven by Malassezia yeast that lives on everyone's scalp plus your skin's individual response to it, and treatment suppresses that dynamic rather than deleting it. Stop treating, and the yeast population and irritation rebuild — often, as you found, within a week or two. DermNet's guidance is explicit on this: medicated shampoos used regularly for at least a month, "and if necessary, indefinitely," with long-term maintenance frequently needed in adults (https://dermnetnz.org/topics/seborrhoeic-dermatitis).
So the fix is a maintenance schedule, not a stronger product. A common pattern: your zinc pyrithione shampoo twice a week, regular shampoo the other washes, then experiment down to once a week if things stay quiet. Two technique details make a real difference — lather it into the scalp (the flake factory is skin, not hair) and let it sit for about five minutes before rinsing; the actives need contact time to work.
If a plateau ever comes, rotating actives helps more than doubling down: zinc pyrithione, piroctone olamine, selenium sulfide, and ketoconazole hit the yeast by different mechanisms, and alternating two of them is a well-worn strategy.
When it's more than maintenance can handle — thick scale, weeping, raw patches, or spread beyond the scalp to eyebrows and nose folds that won't settle — that's a GP or dermatologist visit, since prescription-strength options exist. But for two-week relapse after stopping? Textbook. Resume the shampoo, keep it in the rotation, and it should hold.