The our PC when accessing several websites (no real pattern) blocks popups, despite pop-up blocker in XP being turned off. If you turn it of and on, or on and off then sometimes multiple (3 or 4) clicking on an icon can sometimes bring up the popup.
As good example is tesco's web site when lookig at an item and want additional information so click on the i button, says popup blocked??? This site is not blocked.
You could also try holding Ctrl down before clicking the link that you want to pop up, as this temporarily allows pop ups through. This is useful for sites that need pop ups for forms and don't use them for adverts.