E-mail messages do not show right away

I have a few of my Outlook 2k user complaining that their e-mail messages are not showing up right away. We are running exchange server 5.5 sp 4. I?ve checked and rechecked but nothing seems to be out of place. It is not until they click on something lets say outbox or even a different message then all of their messages show up. If anyone has see or has fixed this problem please let me know thank in advance.