*** Update, May 2020: If you've recently started seeing YouTube ads even with AdBlock installed, YouTube may be testing some advanced ad blocking circumvention on you. We have not been able to reproduce this issue on our end, however, many of our users have reported that signing out of their YouTube account often resolves any recent issues related to YouTube ads, including seeing unblocked ads, the skip button disappearing, or seeing error messages in place of ads. (Sometimes they can sign out and sign back in, but sometimes they must remain signed out.)
This also appears to be happening to users regardless of which ad blocker they're using, which leads us to believe that YouTube is testing some new (particularly aggressive) technology on their end. Unfortunately we can't do much to get around it until they release it more fully, so we can see exactly how they're working around our ad blockers. Rest assured that we're on the case!
There are several reasons you may be seeing ads on YouTube. The steps listed below will help troubleshoot issues related to ads that appear in YouTube videos.
Note: If you’re on a mobile device, we can only block ads that appear on web pages you visit in your browser. If you use a mobile browser that supports AdBlock (Safari on iOS devices and Samsung Internet on Android devices), be sure you are watching videos on Youtube.com instead of the YouTube app.
If you’re suddenly seeing ads on YouTube and didn’t previously see ads in videos before, there may be an issue with your filter lists. A good first step in troubleshooting filter list issues is to perform a manual update: How do I manually update my filter lists?
For AdBlock to block ads on English-language websites, including YouTube.com, you need to subscribe to EasyList.
If you watch non-English videos, also make sure you have subscribed to the filter list for that language. For instance, if you watch videos in French, make sure French + EasyList is enabled in AdBlock's options located on the Filter Lists tab.
Another step that can help ensure AdBlock is working correctly is to clear your browser’s cache and cookies: How do I clear my browser cache and cookies, reset my browser settings, and update my browser?
Disable all your extensions except for AdBlock. Reload the page. Try to watch the video again. If you don't see any ads, then one of the extensions you disabled is somehow interfering with AdBlock. Re-enable each extension one-at-a-time until you find the one that makes ads appear. If you need to use one of these extensions, notify the extension's developers so they can fix the issue that's causing the interference.
If none of the above steps helped, try these additional troubleshooting steps.