Time synchronisation with public time server

Hi,

Our company has several branches across the nation (Australia), hence different "time zone". I want to set each PC to "automatically synchronise with an Internet time server" (located in the Internet time tab). All PCs in the "central location" are able to synchronise with the public time server (128.250.37.2 and/or time.nist.gov) OK...without any problem....(everyone in the company across the nation logs on and access resources from this central site and this is where the firwall, servers are). However, the PCs in the interstates are NOT able to synchronise...and usually come up with the message "error synchronising with time.nist.gov". The sites are connected via "router to router". I am having problem trying to get this to work so any and all help would be greatly appreciated.

I have enabled NTP on our firewall, hence synchronising from the central site has no problem....but no luck from all other states. All client PCs are XP.

Thank you in advance.