Pop-up blockers are not the same as ad blockers. You can have a pop-up blocker that can also block ads within the webpage though.
Some browsers, like Mozilla, have an integrated pop-up blocker but no ad blocker. You need to download an additional plug-in to do that.
So you may have a pop-up blocker on all PC's but an ad blocker only on the first one.
Or maybe it's something else completely.