How is the client getting an IP? Via DHCP? What is the DHCP server? Zentyal? Some router/firewall?
Is the client not connecting to the network or just not appearing on the local dns?
Here, for example, clients get an IP from a DHCP server (not Zentyal), or can be manually assigned one.
The clients cannot register on the local dns directly.
If we need the machines on the dns, I assign a fixed IP and add the hostname + IP to the Zentyal dns.