Some routers will route DNS traffic to Pi-Hole in such a way that all the traffic appears to be coming from the router, and not the indivbidual clients. The Pi-Hole DNS should be given out by the router during the DHCP assignment - check your router settings to determine whether it is set up that way.
An alternative that should resolve the problem is to use the Pi-Hole as DHCP. In this setup, Pi-Hole provides IP addresses and assigns itself as DNS server, and you should then see the individual clients. If the names don't map correctly to what you want to see in the Pi-Hole query logs and other statistics, you can map IP's to plain-text names in the /etc/hosts file on the Pi.