QuickTime recording of FaceTime doesn't record everything

After recording several FaceTime video chats with QuickTime, I clicked on the stop button in the menu bar but all I got was the freezing beachball. I have been able to salvage part of the recording from the Library's Autosave Unsaved folder etc., but it's not the full recording.

Does anybody know why this is happening because I have lost so much information when expecting / thinking everything was going to be fine?

It's an oldish iMac, but I keep it in excellent working order, and is maxed out with 4gb RAM, running 10.11.6

Thanks : )