Security :: Authentication Cookie Expires Too Early?

My users need to stay logged in for 1 day, so I used a persistent authentication cookie with an expiration of 1 day but it still times out after 20 minutes (which is the default timeout for the session, not the cookie).

This is my code:

<authentication mode="Forms">
<forms loginUrl="Login.aspx" protection="All" path="/" />
</authentication>
FormsAuthenticationTicket ticket =
new FormsAuthenticationTicket(1, "username", DateTime.Now, DateTime.Now.AddDays(1), true, "userdata");
string encTicket = FormsAuthentication.Encrypt(ticket);
HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, encTicket);
cookie.Expires = ticket.Expiration;
HttpContext.Current.Response.Cookies.Add(cookie);

I also have manually-generated validation and decryption keys:

<machineKey>
validationKey="......."
decryptionKey="......."
validation="SHA1"
</machineKey>