Return to Sign in Page

It's seem that I'm signed in but every time I click on Sell ,Create a shop, or anything for that matter I get the sign in page. When I do sign in it doesn't say that I entered the password or username incorrectly , it simply asks again. I've tried logging in about 20 times , always get the same problem. I have tried using a different browser and even cleared cookies but no luck. Someone please get to this, it's very annoying.