I'm not sure if this is common knowledge cause I never knew this before but with dandruff, you actually use a non-medicated shampoo to wash out your hair, then apply a medicated shampoo, massage and leave one for at least 5 minutes. You want to do this for a minimum of two weeks, then reduce it to weekly or biweekly based on symptoms.
Since you tried the other agents, maybe you can try a saliciylic acid containing shampoo like Neutrogena or Sevulex. You might want a higher percent. If you don't see results within 4-8 week, you should probably go to your doctor for a prescription strength shampoo