FormsAuthentication Request.IsAuthenticated returns false

I set the Forms Authentication using an authorize attribute that that checks a query string hash value:
FormsAuthentication.SetAuthCookie(qs["name"], false); On the next request I check that the user is logged in:
filterContext.HttpContext.Request.IsAuthenticated This returns false (seems strange so i have a look at the cookie)
filterContext.HttpContext.Request.Cookies< ...