Only the clients that acquire there IP through DHCP will get an DNS entry.
Below example is my Kodi media centre for which I set a static reservation in Pi-hole:
pi@noads:~ $ host kodi
kodi has address 10.0.0.34
Below an example of my NAS which doesnt aquire an IP through DHCP because its set statically, but still I created a static reservation for it in Pi-hole:
pi@noads:~ $ host nas
Host nas not found: 3(NXDOMAIN)
Those clients that have a static IP and dont acquire IP through DHCP can be put in the "/etc/hosts" file on Pi-hole to have them register in Pi-hole.
EDIT: while at it:
pi@noads:~ $ cat /etc/hosts
127.0.0.1 localhost
::1 localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
127.0.1.1 noads.dehakkelaar.nl
10.0.0.3 nas.dehakkelaar.nl nas
pi@noads:~ $ sudo service pihole-FTL reload
pi@noads:~ $
pi@noads:~ $ host nas
nas has address 10.0.0.3