There are lots of possible reasons, but the most likely cause is a browser addon/extension blocking the site from which the icons are served. Try disabling your ad blocker, pop-up blocker, or any other kind of blocker you may have installed to see if that makes a difference.
As for what to do about it if it is one of your blocker addons? You can dig through their lists of blocked sites to see if you can figure out which entry is causing the problem and remove it.
