Two users unable to log on to VPN

We have a pretty simple VPN solution, using the standard Win 2k3 VPN. Basically we have about 15 users with laptops who connect to the vpn, all members of an AD group that is allowed access in the remote access policy.

2 of these users can not log on. They are members of the correct group, and I can’t find anything that would disallow their connection, but no matter what we do they can not log on. When we log on with different accounts from their machines we have no problems, and they are unable to log on from other machines. I’ve checked everything I can think of and can not find any reason why they should have problems.

Anyone have any ideas what could be causing this?