(video) Call Recording not creating .h264 file

I am trying to record the video content of a call, and am having issues with actually generating the video file.

When a call is established i can see mixmon creating the audio (.wav) file, and post call this does indeed contain the audio from the call.

From the posts i have read, the general consensus seems to be that if audio recording works, then the process should create an identically named ( with .h264 extension) for the video RTP data, but this is not occurring, and neither is there any error/warning reported in the full log file.

I am testing between two Bria apple clients, which have ulaw for audio, h264 for video. The Asterisk (1.8.20.1) server is also configured to exclusively use these codecs.
reinvite=no

Is there anything obvious i have omitted/overlooked?

many thanks,
Nick