When logged on at the Gmail web page interface, did you check the Spam folder? It might be that Gmail is incorrectly classifying some incoming messages as spam. If that is the case, those messages will never be downloaded to Outlook on your computer and those messages (the ones in Gmail's Spam folder) will be deleted from the Gmail server after they have been there for 30 days.
The following is a link to the Gmail Help Center, with information on what you can do if some e-mail is being misclassified as spam:
Gmail Help Center, Spam:http://mail.google.com/support/bin/topic.py?topic=12846
This can also happen with other e-mail providers (e.g., the e-mail address provided by your Internet Service Provider). Some ISP's and e-mail services allow you to disable their spam filters, while others do not.
If you use an e-mail client program (like Outlook or Outlook Express) to retrieve your e-mails, it is a good idea to occasionally log on directly to the web-based e-mail interface (for each e-mail address you have) to verify that legitimate e-mails are not incorrectly classified as spam at the ISP/e-mail-server level.