Why don't my teeth whiten?

This is kind of a silly question, but I'm really troubled. I drink coffee and I know that can contribute to the yellowing of teeth but I only drink a cup a day. I brush three times a day with a whitening tooth paste, use a mouth wash, floss and I've even tried white stripes! My teeth just won't get any whiter. I don't drink soda or anything like that, just coffee. I know lots of other people who drink more coffee than I do but still have brilliantly white teeth so I dunno! What's going on and why won't my teeth get any whiter?