It is impossible to determine why automatic polling is failing when you do
not post your mail account type, connection type and polling settings. If
automatic polling once worked and now fails, you also need to examine what
changes were made to your setup before automatic polling stopped working.

In general, here are the reasons automatic polling might fail in Outlook:
1. You have created an unrealistic polling interval. Many users set their
polling interval to 1 minute thinking they will be downloading mail
continuously as it arrives. Such settings are not recommended. They put an
unnecessary strain on mail servers and can create conflicting polling
requests that can damage your Send/Receive Group or even your account
settings. 10 minutes is the recommended minimum.
2. While exiting Outlook, you inadvertently set your profile to Work Offline
(this option is immediately above the Exit command in the File menu).
3. You have a damaged Send/Receive Group as documented here:
http://support.microsoft.com/support/kb/articles/q312/3/36.asp
4. You have overridden your Send/Receive Group settings as documented here:
http://support.microsoft.com/support/kb/articles/q320/8/36.asp
5. Your mail account settings are corrupt. Recreate your mail account from
scratch.
6. Firewall software is blocking your transmission. Disable your Firewall
temporarily to see if automatic polling is restored.