I'm a bit stumped then! The only think I can think of is that the WAN dynamic IP is changing, but you say that
www.whatismyip.com is not showing a variable IP.
I do exactly what you're trying - with a Linksys Web-cam - but use a DynDns service (
www.dyndns.com) to track my dynamic WAN IP and provide a domain name to map to the camera over the web.
Also to get to the web-server in the web-cam I had to re-configure the web-cam to
not use port 80, as I already have a web-server on the same LAN employing port 80; so I deliberately moved the web-cam off unto an obscure port to avoid the clash. I don't know if this is a consideration for you.