If Pi-hole is used then you can see that the logs are appearing. That implies that those client web browsers are not using Pi-hole for DNS, even if the OS is. Try that nslookup test on each client to confirm it's blocking flurry.com and that you can see the query in Pi-hole.
Then try navigating to flurry.com in the browser. You should the browser's "unable to connect" error and see the query in Pi-hole's log as blocked. If you see the site load, then the browser is not using Pi-hole. Probably a plugin or setting using another server or DoH in the Firefox example I gave above is ignoring the OS's DNS and bypassing Pi-hole.
You may find that your mobile has additional DNS servers in use. The OPPO and OnePlus are known to do this, see this post for info. If non-Pihole DNS are configured then the computer or device will always be able to bypass Pi-hole.