I have an ashx file (implementing the IRequiresSessionState interface) storing a value to session state like so:
context.Session.Add(\"answer\", _answer);
I susspect that the issue is on url name and in the setup of your cookies on the session, because if you have place the IRequiresSessionState interface you sould read the session.
Go to your web.config and set your domain name on the httpCookies as your name of your domain with out the 'www'.
<httpCookies domain="domainname.com" .... />
This way your page reads the cookie and the session ether you call it from domainname.com, ether www.domainname.com.