Pptp virtual server IP forwarding

I have a pptp virtual server in which I am using fast l4 as of now.users will be connecting to my virtual server from which the traffic will be sent to a VPN server for the users to connect. Here users will sening an GRE packet to connect to the VPN server the problem which I am facing is only one user can connect no other users can connect is it because of the selfip which will initiate a connection to VPN server and VPN server cannot respond to same IP for multiple requests ?? Or is there anyway I can forward the user IP directly to the VPN server for this traffic .kindly help me in this scenario . TIA