Please try taking following measures:
First of all, update filters by right-clicking the extension icon and choosing Check for filter updates.
Click on the extension icon and then on the gear icon in the top right corner. Go to the Filters tab and make sure that "AdGuard Base" filter is enabled there. Same for language-specific filter(s) for your language. If needed, add them by clicking the Add a filter button.
If you are using a Chromium-based browser, install AdGuard Extra
browser extension. It helps block ads in some difficult cases when the regular extension alone isn't enough.
While on the problematic website, click on the extension icon and select Block ads on this website to block the ad manually. Then report the missed ad by clicking Report an issue in the extension menu.