I have clients that connect to a Window Server 2008 R2 box via PPTP. They are able to successfully negotiate a connection and receive a local IP address, but are unable to access any internal network resources, nor access the Internet when connected.
There are no error messages or logged messages indicating a problem.
The problem is most likely that you haven't configured routing properly in RRAS. There is guidance in the documentation at http://technet.microsoft.com/en-us/network/bb545655.aspx
Have you enabled "LAN Routing" in the RRAS configuration?