Session data and 304 Not Modified HTTP code

I am storing an id in the session data. An action causes the id to
change, however, the rendered action has not changed. The server
returns a 304 Not Modified code and doesn't return the cookie. With
Rails 2.2.2, I believe it is using the CookieStore for sessions, so I
expect the cookie to update.

When I hit the site again, the old session data is used. Is there a
way to use the new session data?