It may be an allergic reaction to the same anti-dandruff shampoos, no kidding. Try to wash your hair for a while with some hypoallergenic shampoo (most likely, there are such among children). The result will not be noticeable immediately, only after a week and a half, because the skin needs time to calm down and recover.
But there may be other reasons. Allergic reaction to any food product or flavoring additives, to medications and even to vitamins, if you are taking any.
Dandruff can also pester because of mental strain, stress, lack of sleep, as well as hypothermia scalp (in winter)
The best remedy is hellebore water (sold in pharmacies, at least not long ago. However, drugs that "work" systematically disappear from sales for obvious reasons ((((((((No shampoos) stood !!!
A good way is to have a haircut on your bald head, and you will save on shampoo and there will be no dandruff. And also you do not have to waste time on hairstyles and trips to the hairdresser