Please try taking following measures:
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.
While on the problematic website, click on the extension icon and press the [Select] button next to "Block an element" text to block the ad manually. You can also report the missed ad by clicking [Report] in the extension menu.