Session variable lost

[hdsllc]

I have an ASP.NET 2.0 app where I have the following scenario:

1. Set a session variable in page A.
2. From page A, click a link to navigate to page B (session variable needed in page B).
3. Click refresh button on toolbar in page B. Error occurs because session variable has been lost.

This occurs in VS 2005 dev environment (VS web server) or production server.

The sessionState section of my web.config is as follows -
<sessionState
mode="InProc"
cookieless="UseCookies" cookieName="ASP.NET_SessionId"
timeout="20"
/>

Any ideas?