Unable to share desktop in Netmeeting

I have a few computers is unable to share the desktop in Netmeeting. The
"Remote Desktop Sharing..." is alway gray and also Remote Assistance doesn't
work for these computers. However Remote Desktop works.

I checked the following:

Both allow Remote Assistance and Remote Desktop are checked.
The following Group policies are the same as other computers without such
problem:
Allow logon through Terminal Services (Not defined)
Deny access through Terminal Services (Not defined)
Netmeeting Remote Desktop Sharing (Not defined)
Disable remote Desktop Sharing (Not defined)
Remote Assistance is also on Firewall exception list.

I uninstalled Netmeeting and reinstalled it again. It still doesn't work.

I appreciate anyone provide solutions or hints.

Thanks,

James Yang