That will be because of silence detection. The DVR is made for recording premium TV shows and movies from services such as netflix and it will stop if it doesnt get some audio within 30 seconds (by default). Settings > DVR, and set "Stop if no sound" to something really large if the cam broadcast is quiet.
Cam4 can be captured in AUTO mode btw, no need for DVR unless you want the chat window as well.
recording always stops after 30 seconds. I tried to reset registration and turned off auto-stop after silence but it doesn't work.