Email protection is not consistent

I have Internet Security 2013, version 20.4.0.40 on a desktop PC with Windows XP and using Outlook 2002 for my email.    I have noticed that my email protection is not consistent.    Some emails are being sent to the Norton AntiSpam folder but others are not.   In some cases, there are duplicates - one in the AntiSpam folder and one in the Inbox.   I have been "teaching" the system which emails are spam and even get the message stating that it is learning.    Is this a known problem?   Why would there be inconsistencies?    If you need additional information about my system, please reply and let me know.   Thanks in advance for your help.     Kris