Why do I still have dandruff?

I have been using “head and shoulders: dry scalp care” shampoo for a few years now. I wash my hair every day and that has kept the dandruff away, until a few weeks ago. I haven’t changed much in my routine. I’m at college now and my diet’s a little different than 2 months ago, but this didn’t happen at college last year. I’ve also been occasionally wearing a hat, but it’s not like an every day thing.
Anyone know why I still have dandruff?