Issues with Domino Web access

We have enabled the session based authentication with a HTTP session timeout of 30 minutes ( In server document).

However some of our users who are using IE 7 or Firefox 3 their session is getting timed out even before 30 mins and server asks for re authentication. In some cases their session is expiring even in 3 mins.

Can anyone help me how to troubleshoot this issue??

Thanks in advance