Cisco ASA VPN Client Authentication

[Lukey]

I have an ASA 5520 setup to receive VPN connections using AD authentication which works fine. The problem is, although the user is authenticated to the ASA, this is not passed onto the Domain so that when they go to access a restricted share that they should have access to, they get 'Access Denied'. I must be missing something in the setup but if anyone can help, that would be great.

Thanks