Connecting to a VPN with wireless

We have two laptops that connect to a VPN using Netscreen Remote. They are
connected to an ADSL Router by wire and connect to the remote server through
the VPN with no problems. However what we can't do is connect to the VPN
using WiFi. We have tried with the wireless connection on the Router and
tried connecting with Wifi in hotels but cannot get the connection to work.
Incidentally we can connect to the Internet with WiFi and the Netscreen
Remote icon in the task bar displays a yellow key which normally indicates a
connection but we can't log on to the remote server.
Anyone any ideas why we can connect with a wired connection but not with a
WiFi connection?
Thanks
Tony