Windows 2003 L2TP/IPSec problem

ran through the microsoft tutorial on setting up a L2TP VPN connection.

i installed certificate services on the 2003 box and i ran through the
instructions for auto enrollment. i verified that the certificates were
uploaded to the two workstations.

however, when i try to establish a L2TP/IPSec connection, after a long delay
the connection times out.

here's the only error message i can find on one of the client's event
viewers:


--------------------------------------------------------
Event Type: Failure Audit
Event Source: Security
Event Category: Logon/Logoff
Event ID: 547
Date: 5/2/2004
Time: 9:05:34 AM
User: NT AUTHORITY\NETWORK SERVICE
Computer: SATURN
Description:
IKE security association negotiation failed.
Mode:
Key Exchange Mode (Main Mode)

Filter:
Source IP Address 192.168.x.x
Source IP Address Mask 255.255.255.255
Destination IP Address 172.16.x.x
Destination IP Address Mask 255.255.255.255
Protocol 0
Source Port 0
Destination Port 0
IKE Local Addr 192.168.x.x
IKE Peer Addr 172.16.x.x
Peer Identity:

Failure Point:
Me

Failure Reason:
No response from peer

Extra Status:
0x0 0x0
----------------------------------------

any ideas on this one?