slidingExpiration not working for auth cookie

Hi there

We have a login page that manually creates a cookie.  In the web.config we have set slidingExpiration="true" - however this (sliding expiry) is not working (the users session becomes invalid after the specified timeout (30minutes) from their original login time.

For now we are working around this by manually resetting the cookie upon every page request - which is not ideal.

Any ideas as to why this is occuring and how we can fix it?

The code that creates the cookie is (in the login page after authenticating the user):FormsAuthenticationTicket fat = new FormsAuthenticationTicket(1,
sessionId, DateTime.Now,
expires, false, "",
FormsAuthentication.FormsCookiePath);
Response.Cookies.Add(new HttpCookie(FormsAuthentication.FormsCookieName, FormsA ...