Why some sites still show ads with a blocker on
Blockers work from lists, and some ads never match a list. Server-side ads, first-party scripts and in-video ads are built to look like the page itself — here's which ones a blocker can reasonably catch.
An ad blocker doesn't understand what an advert is. It works from filter lists — long, community-maintained lists of addresses and page patterns known to serve ads or tracking. When the browser tries to load something on a list, the blocker stops it or hides it. When something isn't on a list, it loads like any other part of the page.
So when ads get through, it's usually not that the blocker has failed. It's that the ad has been delivered in a way that makes it indistinguishable, by address or pattern, from the content you came for.
What a blocker catches easily
Most online advertising is still served by third parties. The page you're reading asks an ad network's server for an advert, and that server's address is well known. Filter lists cover these networks thoroughly, which is why a blocker removes most banners, pop-ups and sidebar ads on most sites with no effort from you.
Tracking scripts work the same way: they load from recognisable third-party domains, and blocking them is straightforward. This is also where much of the speed benefit of a blocker comes from.
The ads that are built to get through
First-party ads. Some sites serve adverts from their own domain, through the same server that delivers the article. Blocking the address would block the site. Filter lists can sometimes target these by page structure instead — a particular box in a particular place — but site owners can change that structure at will, and lists take time to catch up.
Server-side ad insertion in video. Many streaming services and video sites now stitch ads directly into the video stream before it reaches you. There's no separate ad file to block: the advert and the programme arrive as one continuous stream from the same server. A blocker can't remove it without removing the video.
Native and sponsored content. "Sponsored" articles, promoted posts in social feeds and paid product placements are written into the page as ordinary content. They're labelled as ads, but technically they're just text and images from the site itself.
In-app ads. A browser extension only works inside the browser. Ads in phone apps, smart TV interfaces and games aren't affected by it at all.
A quick reference
| Ad type | How it's delivered | Can a blocker catch it? |
|---|---|---|
| Banner and display ads | Third-party ad network | Usually, yes |
| Tracking scripts | Third-party domains | Usually, yes |
| Pre-roll video on many sites | Separate ad request | Often, though sites fight back |
| First-party ads | The site's own server | Sometimes, by page structure |
| Server-side inserted video ads | Stitched into the stream | Rarely, if at all |
| Sponsored posts and articles | Part of the page content | Sometimes hidden, not reliably |
| Ads inside apps | The app itself | Not by a browser extension |
Things on your side worth checking
- Is it switched on for this site? Most blockers let you pause them per site, and it's easy to allowlist a site once and forget. Click the extension's icon on the page to check.
- Are the filter lists up to date? Lists update automatically, but you can usually force an update from the extension's settings. New ad techniques are often covered within days.
- Is an "acceptable ads" setting on? Some blockers ship with an allowance for ads that meet certain standards, switched on by default. It's a legitimate choice, but if you'd rather see none, look for it in the settings.
- Which browser are you using? Chrome's current extension platform places limits on how many filter rules an extension can use and how it can update them. Blockers still work, but some are less thorough than they were. Other browsers have built-in blocking or allow more capable extensions.
Anti-blocker messages and what to do about them
Some sites detect a blocker and either nag you or refuse to show content. That's the site's choice, and your options are simple: allowlist the site, use a different source, or pay for the site's ad-free option if it offers one.
Where else blocking can happen
If in-app and smart TV ads are the ones that bother you, a browser extension won't help. Network-level blocking — a DNS filtering service, or a filter set up on your router — works across every device on your home network, including apps. It still relies on lists, so it has the same blind spots for server-side and first-party ads, and it occasionally breaks a legitimate app that shares an address with an ad server. It's a trade-off rather than a complete answer.
The realistic goal for any blocker is to remove the bulk of third-party ads and trackers. It does that well. The ads that remain are the ones built into the content itself, and no list can separate those cleanly.