XPpro SP2 to XPpro SP2 hostname access

I have a network with a PDC running a domain, it also runs DHCP and WINS and passes all traffic to my firewall which handles proxy and DNS. It works, is fast and stable.

On this network I have an XP pro SP2 PC with a share on it with the security set to everyone = full access. From any other XP pro SP2 pc in the company I can access the share by its ip e.g. \\192.168.1…\Share but not by its host/pc/machine name e.g. \\mypc\Share.

All Pc’s are exactly the same configured. Netbios also switched on on all of them.

Any ideas welcome please! Its just so bizarre because technically there areno reasons why this shouldnt work.