If you are using IE and you click on a campaign banner at the bottom of a scrollable page, you may not see the lightbox unless you scroll back up to the top of the page.
This is browser specific issue due to the page not including a DOCTYPE declaration in the HTML. If a DOCTYPE is not declared, the lightbox function is unable to determine where to position itself within the browser.
Add the appropriate DOCTYPE tag as the first element within your site.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
Read http://www.w3schools.com/tags/tag_doctype.asp for more examples.