Gmail uses a common timing algorithm to fetch messages from POP accounts, one you can somewhat game, but sometimes you want to get at a message you know has arrived. A new Labs feature adds a manual refresh button for POP grabbing.
Head to your Labs settings and enable the “Refresh POP accounts” lab, then hit “Save changes” at the bottom or top of the page. From then on, you’ll see a “Refresh” link to the right of your inbox buttons. Hit it and Gmail will poll any email accounts you’ve set it up to fetch from. Want to get started importing your other mail accounts into Gmail? Check out Gina’s guide.
New in Labs: Refresh POP accounts [Official Gmail Blog]