SSO problem with ESS

Hi,

We have implemented ESS portal as an intranet application with SSO feature. Most of the employees are able to access the ESS portal via SSO. However there are a few employess that are having problem with SSO. They are always prompted to key in their login id and password whenever they try to access the ESS portal. Anyone encounter similar problem and how do you overcome it?

Thanks & regards,

TE