What determines NetBIOS names in ARP Cache?

When I look at the ARP Cache, some IPs have the NetBIOS name, and others don't.<br/>
As far as I know, all the PCs on our network have the NetBIOS name, but not all show in the Vigor ARP Cache table.<br/>
Can anyone tell me what might be the cause? It would have be useful to have them all appearing.