Notes/Domino 4 and 5 Forum

We are experiencing login problems with our clients on our website. Some clients are successful but the clients that are not successful, they can only login sometimes. We have a Thawte Certificate that we have checked over and over, and that does not seem to be the problem. We have also looked at Maximum Active Sessions but that is at default like it always been. Our application never had a problem before until recently, our application has been live for over 3 years. Does anyone have any ideas what could cause this? PLEASE HELP!