On certain pages, Adblock Plus may not work as expected. Clicking the ABP toolbar icon may show that there is nothing to block, or opening the extension manager (the puzzle icon on the toolbar) may indicate that the extension is not allowed on that page.
This happens because "internal" pages in all browsers, including Chrome, Firefox, Edge, and Opera are not accessible to extensions. Internal pages include the New Tab page, the extensions manager, the browser's settings, and so on. Browsers don't allow extensions to inject code into these pages because these pages can change the browser behavior. Allowing extensions to possibly co-opt the browser would be a security risk.