Are you someone that uses their browser’s private browsing mode? In Google Chrome, this is called Incognito Mode. In Edge, it’s called InPrivate. And for Firefox, it’s just called private browsing.
If so, it’s important to know that your browser’s private browsing function typically disables browser extensions. This is because extensions are often made by third parties and often involve tracking your browser history.
If you want to use Adblock Plus while browsing in private mode, it’s easy to enable.