Hi ,
Clients must generate some kind of traffic to be listed in the network-wide clients page. There used to be a documentation page about that , but I can't find it. It used to state that it must send 'web traffic' or something like that. Intra-vlan traffic wasn't enough to map the client.
The ARP table however is essential. It should always be valid and containing the clients of that broadcast domain.
Edit : I was typing this while @Brash had already found the documentation. 🙂 I'm a slow typer haha