OEI SSO Login Issue

I have a client that is using SSO with OEI.  Some of the users are able to login using SSO without and issue.  Other users with the same Role and Privileges cannot login.  When they click the Send and Add to CRM button and are asked to authenticate to CRM OD, they click on SSO, input the CompanyID but are redirected to the standard user login page.  I suspect that these users may not have properly installed OEI for SSO; however, I am not sure.  If this is the case, how can the issue be corrected WITHOUT having them uninstall