Series Recording not recording all shows

I have created a series recording for Mythbusters. I selected First Run/Repeat, and all show times, on the Science Channel and whatever other channels it shows on. However, the DVR seems to be recording random shows, certainly not every time it's aired. I have encountered this problem in the past, and not just with Cox, or with this show or channel. It also occurred with our AT&T Uverse DVR. There is plenty of space left on the DVR. Has anyone else had this problem, and is there another setting parameter I'm not aware of to ensure that all showings are recorded?