Error 720 with VPN Connection

Sorry if this is asked a lot - I'm a software guy who's been lumped with a
hardware guy's work this week...

We have set up a Windows Server 2003 VPN using RRAS. It is behind two
symantec firewalls and an ADSL modem/router and we have configured the
forwarding appropriately. After a little bit of mucking around, we have it
working beautifully...except that not all machines connect successfully and
there appears to be no pattern that would explain why.

The first machine we attempted to use was an XP Pro laptop on the site using
dial-up - this one froze at the Checking Username and Password... dialog for
a while before reporting 'Error 721 The Remote Computer did not Respond.'

The next machine was at our offices which have an ADSL connection and that
one connected immediately without a hitch every time.

I am now at home on the couch with a Laptop to the left of me that connected
immediately without a hitch, while the one on my lap stalls with the same
eventual Error 721. They're both using the same wireless access point, so
it's not related to the network.

The two machines that connect are a Windows XP Home SP2 and a Windows XP Pro
SP2. So far there are three that don't, two Windows XP Pro (both SP2) laptops
and my Windows XP 64bit desktop.

All the firewalls have been turned off and it makes no difference. In the
event log od the VPN server the connections result in a warning in the event
log with a 20209 Event ID from Rasman.

I really hope someone can help me here, is this a common problem?

Thanks in Advance,

Dave