Been having same issue and found couple things.
On most of the shows it happens with the start times are 1 minute off. I've programmed a show thru series manager that could be 8:00-8:30, 8:30-9:00 or any other time. But the following weeks show or 2-3 weeks later the start time switches to 8:01-8:31 or 9:01-10:01. That weeks show doesn't get picked up to record. So I have multiple shows listed for the same show, one has normal start times on the hour or half hour and others with a minute late for start and end times.
Again 1 week the show starts on hour and next week starts 1min late you'll need multiple recordings in series manager.