Easy one: Why doesn’t Outlook pull all my pop3 email?

I use Outlook 2003 to pull emails from 3 email address that are gmail based. I’m starting to notice that OL doesn’t pull everyone of them. Its random, but there are emails, not spam related in my main gmail account that it doens’t pull down into OL. Any ideas? To my knowledge these emails in gmail weren’t read before on their server, as they show up new and unread. OL just doesn’t pull them. Thanks in advance!!!