User.IsInRole("<somegroup>") returning false????</somegroup>

[pnslcs]

Howdy, I have had a web site running for some time now and I am constantly having issues with the statement User.IsInRole ("somegroup") returning a false value. It does not always happen but often the user is a memeber of the group which membership is being tested but the statement returns false. The application makes decisions based upon the return value and then the user does not have proper access to certain functionality within the site. We normally have to stop and start the application pool on the iis server. Is there anything I am doing wrong? thanks Patrick