One possible reason is that IP Forwarding may be disabled (Control Panel / Network / Protocols / TCP/IP / Properties / Routing).

When IP Forwarding is enabled, Windows NT routes IP packets using the routes defined in its' route table. The RAS server routes IP packets from its' RAS clients to its' LAN (even if they exist on the same subnet).

When IP Forwarding is disabled, the RAS server no longer routes IP traffic, even when the RAS server is set to permit RAS clients Access to Entire Network.