These tools can help:
Ads can be seen for a number of reasons:
(1) The client is not using Pi-hole for DNS. This can be due to improper router DHCP configuration, availability of an alternate DNS server to client ("secondary DNS" in router, or IPv6), or use of a VPN service on the client.
From a client that you believe should be connected to the Pi-Hole for DNS, from the command prompt or terminal on that client (and not via ssh or Putty to the Pi), what is the output of
nslookup pi.hole
(2) The browser in use on the client has private or secure DNS enabled, and the DNS requests from the browser aren't going to Pi-hole.
(3) The ads cannot be blocked by Pi-hole. This is typical with YouTube and Facebook, but does not appear to be the case for you.
(4) The client(s) are in a Pi-hole management group with no or limited blocking.