Ads are blocked on a website or YouTube channel where you have told AdBlock to allow ads to appear.
Why is it happening?
The short answer is that it's an unintended consequence of a web technology websites can use to give their visitors a better browsing experience. More and more sites are using the technology, and that's why we're seeing this issue reported more often now than in the past. If this is enough information for you, feel free to skip down to "What can I do about it?"
The longer answer is a bit technical, so bear with us for a moment. A web page is made up of text, images, and scripts (little pieces of software that perform specific functions on the page such as submitting a form or requesting ads). Normally scripts run in the foreground. That means that when a script is running (after you click "Buy" on a shopping site, for instance), the page stops responding until the script is finished. This obviously doesn't provide a good browsing experience.
Enter web workers. Web workers allow scripts to run in the background, where they don't interfere with your interaction with the page. Using our example above, when you click "Buy" on a shopping site that uses web workers, you can continue browsing while the script runs. Web workers provide a much better browsing experience.
Web workers also allow websites to share scripts between tabs (for instance, when you have several pages open on the same site). However, they don't identify which tab generated a particular script. As we noted above, one of the functions scripts perform is requesting ads. Ad blockers have no way to know which tab generated an ad request. That means they can't determine that the request came from a page or site you whitelisted and should be allowed, not blocked.
Whitelisting in AdBlock still works well on sites that don't use web workers. But as more sites begin using them, we're starting to see more reports that whitelisting is failing on those sites. We are investigating the issue, but it's proving to be difficult to solve.
What can I do about it?
The easiest way to allow ads to appear on a whitelisted site or YouTube channel is to pause AdBlock. Just remember to unpause it when you're finished!
To pause or unpause AdBlock, click the AdBlock button in the browser toolbar and select Pause on this site or (if it's paused) Resume blocking ads.