There are a few reasons why you might be seeing ads despite the ad-blocker being enabled.
First off, some ads are simply baked into a website itself and can't really be blocked automatically. The ad in this case is treated like any other images or media on that website so there's almost no way for an extension or DNS filter to identify that image as being an advertisement.
If you are specifically using the ROBERT DNS ad-blocking, some ad domains will load from the same source as the website resources making it impossible to distinguish between an ad and legitimate content. A common example of this is Youtube video ads which load from the same domain as the video itself. This is why we recommend using our browser extension's ad-blocker in conjunction with the desktop app as the extension can differentiate between the ad and the video due to its access to the webpage's DOM.
Second, you might be running into an ad domain that has just recently been created and deployed online. New domains for ads are created every single day and ad-blockers, while incredible at keeping those domains at bay, aren't able to determine immediately that a new domain is one for ads. After a few days, you might see that ad disappear as community-sourced ad domain lists (which Windscribe uses several of) are updated with the new domains to block.
Third, make sure the ad-blocker is actually active and there are no exceptions for that site. You may have added a whitelist rule for a site in the past and forgotten about it only to come back and see ads there. Removing the whitelist rule should fix the issue in this case.
If you continue to see ads for a while when using Windscribe, let us know where exactly you see them in a support ticket and we'll look into it: https://windscribe.com/support/ticket. Provide the links to the page and screenshots if possible.