VPN NAT Transparency

Hi,

I have set up a Windows 2003 VPN server. I'm using L2TP/IPSEC. VPN is
funcioning well. But I have one specific problem.
NAT-T is also working but only one in time. When several clients behind the
same NAT device try to connect to VPN server, only one connection to VPN
server is made and working. The other clients gets "Error: the remote
computer did not respond".
Can Windows 2003 VPN server serv also multiple L2TP/IPSEC connections behind
one NAT device ?
The scenario :

3 VPN Clients - > NAT -> internet -> Win2003 VPN Server

The only working scenario now is following :

1 VPN Clients - > NAT -> internet -> Win2003 VPN Server

Thanks.