Hi, probably it is because of seborrheic dermatitis, you might be having acne problem. In some people, a yeast (fungus) called malassezia grows in the sebum along with bacteria. Antifungal treatments, such as ketoconazole (Nizoral), are often effective.
“Try using the shampoo daily until your symptoms are controlled, then cut back to two or three times a week. If one type of shampoo works for a time and then seems to lose its effectiveness, try alternating between two types of dandruff shampoos. Be sure to leave the shampoo on for at least five minutes — this allows the ingredients time to work.
If you've shampooed faithfully for several weeks and you're still experiencing an itchy, flaky scalp, talk to your doctor or dermatologist. You may need a prescription-strength shampoo or more aggressive treatment with a steroid lotion”.
Taken from website http://www.mayoclinic.com/health/seborrheic-dermatitis/DS00984/DSECTION=treatments%2Dand%2Ddrugs
I have it too, and I can't get rid of it. The only thing that the Dermatologist has prescribed that has helped some is Clyndomicin.