Sounds like the clients that you are trying to blocvk the adds on are still leaking their DNS requests through to your ISP. I would recommend setting your routers DNS to youir pihole and having the PiHole do the rest. Failing this, if your ISP's router does not allow forwarding DNS requests to a custom server, Follow the advice from step 3 HERE:- https://linustechtips.com/main/topic/1094810-pi-hole-setup-tutorial/
Linus did cover this section on a youtube video, but in basic terms, if your ISP provided router/AP does not allow setting custom DNS servers, disable the DHCP option on the router and enable the same setting on the pihole. This will enable PiHole to act as a DNS resolver for all your divices on your network without having to manually change settings on a device by device basis.
I've done this myself, and it's as simple as changing a couple of settings.
Sould be easy enough!
P.S, There is a second helpfull article HERE:- How do I use Pi-hole's built in DHCP server (and why would I want to)? on how to enable the DHCP server on the PiHole and disabling it on your router. In simpler terms, your PiHole becomes your DHCP server and DNS resolver, NOT your router.