VPN Authentication

I have an ISP provider, they have given me 4 public address. I have added the static mapping in my Pix firewall to my vpn server. I have opend TCP, UDP and GRE ports on the firewall. I get authenticated through my ISP but when i try to authenticate to vpn server it fails. The VPN server is a child domain controler. I have granted the user the rights to VPN and RAS through Active Directory. Any ideas where my problem could be. I also tried to VPN on my network to test the VPN. It works on my root domain, but fails on the child domain. Hopefully someone can help. Thx