Outlook Plug-in Auto Sync

We have installed the Outlook Plug-in and configured it to Auto-Sync. It begins syncing older messages and then it stops. Newer messages do not sync. I can manually sync one message at a time, but the Auto-Sync is not working. Is there a service in the background that needs to be restarted if Auto-Sync isn’t working? Could another service or application be causing a conflict (anti-virus, firewall, etc.) Any other suggestions?