Good, so it has renewed its lease with Pi-hole and is using it for DNS.
The client device OS as well as certain software as browsers may employ their own DNS caches, which could have them hold on to already known IP addresses from previous DNS lookups until the TTL for a domain expires.
Try if clearing those caches would change your observation. If unsure how to do this for a specific device or software, a restart of a device should take care of this.
If the issue still persists after clearing caches, try to find out which domains are serving the ads:
Note that Pi-hole as a DNS blocker can only block domains.
If a website would serve some of the ads from the same domain as its content, Pi-hole wouldn't be able to block it, unless you were willing to also forfeit the content.