Major Dandruff

Hey everybody (: I was just wondering what you think I should do with my pony Shadow; she has really bad dandruff in her mane and tail. Its not even dandruff anymore in her tail, its flanky chunks of skin. And she's beginning to loose her mane, strands are just coming out when I run my fingers through it. I've tried normal shampoo and anti-dandruff - that seemed to help for a couple of days, but then it came back. What should I do?