asterisk users - Jun 2008 - MixMonitor Not recording whole calls

I have calls being recorded via mixmonitor which are not being recorded 
in their entirety.

The calls are incoming G.729 calls recorded in G.729 format (which I 
know means a lot of licenses, and a bit of runtime, but the load on the 
server isn't great and it does save disk space).

They seem to stop recording if the call it placed on hold for an 
extended period of time.

Does anyone know what is happening?