If my memory serves me well, it is a old problem of some polycom and ffmpeg and [Open]H323[Plus], about few flags on packets, unsolveable cuase polycom implementing parts of codec and packetizer on hardware. you can find clues searching on old mail archives on the 1st verions of OpenH323 (the old project).

2010/5/13 Pablo Sotomayor pablo@sotomayor.com.br:

I just notice another problem with H263 video. When using openmcu with polycom pvx clients, the video is ok when 2 endpoints are connected, when i connect a third pvx software, the video freezes but the audio keep ok. Openmcu and endpoint seems like send receiving data/audio/video. When i connect a fourth pvx endpoint, it shows the frozen video and audio is ok. stats of send/receive data shows traffic too. When i disconnect the second and third endpoint, the video unfreeze and i got video on 4th endpoint. Any help with this H263 issues will be appreciated. Pablo