Thanks for the query.
Severe dandruff could indicate dandruff itself which is significant or can indicate beginning of some other skin condition like Seborrheic dermatitis or Scalp psoriasis. Kindly consult a dermatologist for exact diagnosis and treatment plan. It is also important to prevent their relapse once you have got it cured.
Next Steps
You should keep the shampoo at-least for 5 minutes on the scalp for it to act. This should solve your problem. However if problem persists, consult a Dermatologist for proper diagnosis and treatment and to rule out any other skin condition. Hope it helps you.