What Causes Itchy Scalp After Applying Neutrogena T-Gel For Seborrheic Dermatitis?

Yes I was recently diagnosed with seborrheic dermatitis and the doctor prescribed me nizoral shampoo and recommended neutrogena T-Gel as well I use both of them every three days as told to but my crown is just as itchy as dry and I don't use a lot of the shampoo what's the problem?