On Linux and Mac client PC's, whats outcome for below one ?
Or if its Windows client PC, what does the DNS section display when run below one in a CMD prompt ?
Like @Therion87 mentions, you want clients to query the Pi-hole IP address directly and not via your router first.
You can test DNS lookups and blocking on a Linux/Mac/Windows client PC with below ones:
nslookup doubleclick.com <PIHOLE_IP_ADDRESS>
nslookup doubleclick.com 188.8.131.52