pidgin to gmail: voice both ways, video one way only

I use pidgin to connect to my MXPP OpenFire server, and chat with users on other XMPP servers such as gmail. An audio/video chat to a gmail user works consistently, I see their video, but they don't see mine. Audio works fine both ways (with headset :-). I have done a few tests:

pidgin on Ubuntu - OpenFire - firewall - gmail server - gmail plugin on Windows | audio both ways, video Windows --> Linux only
pidgin on Ubuntu - OpenFire - pidgin on Ubuntu | both audio and video work fine, both ways
pidgin on Ubuntu - OpenFire - firewall - gmail server - pidgin on Ubuntu | both audio and video work fine, both ways

The one thing that is definitely different is that it seems that when I use pidgin to connect to the gmail server, the session seems to go through the servers, while when I chat with somebody with the gmail plugin on Windows, it seems to establish a peer to peer connection (looking at the output from "lsof -i"). I have tried to kill all my rules in iptables on the server, and that doesn't make any difference.

Any idea ?

Thanks.