hi - I have had the same problem with 13 year old DD. She had greasy hair the next day after washing it with flaky scalp. Took her to GP who said it was hormones but to try Tgel and towel dry hair as much as possible after washing. This GP was a lady and said if the dry the roots of the hair it will tend to de-grease it.
Also advise was not to let child put her hands through her hair!
Anyway seems to have worked - DD now washes her hair every 3rd day approx and it looks a lot better.
Diluting the shampoo a bit a in a jug or cup helps to get the shampoo to all parts of the hair. dd can be a bit rubbish at hair washing, but this has helped her- we used to find bubbly bits left in her hair.
Head and shoulders citrus is very good as is 'I love juicy' from lush.