Portal page SSO not working with Webdispathcer

Hi

We have our portal system connected to CRM system at backend

When i log on the system through http://hostname:50100/irj/portal it logs in and does not ask for authentication for CRM .which confirms that SSO between portal and CRm system is working

But when i access it through(web dispatcher) http;<webdispatcher>/irj/portal it logs in but asks for CRM credentials as well ...this means some place SSO is not working.

Can you please let me know why SSO does not work for web dispathcer?

Regards

Tushar