1. Can you ping the machines IP address when the VPN is connected?
2. Can you access the share via UNC path? e.g. (Windows) Start menu --> run --> \\192.168.0.1\share
3. Do any of the machines that you cannot connect to show "Not Accessible" next to its name?
4. Is the Local Participation on the host set correctly?
5. Is NetBIOS over TCP/IP enabled on the external adapter for this machine? Also, is file and print sharing enabled?
6. Are you running any security suites? Nortons, ZoneLarm, Mc Affe etc..
Some more troubleshooting is available at the bottom of this post:
http://forums.qbik.com/viewtopic.php?p=16108#16108
Please let us know how you get on.