Could be a number of things. Was the timer set for new episodes and the missed one was a repeat? Timer conflict? Press the DVR button three time, then page up to see what it says for the specific program timer...ie, skipped, not a new event.
Can you scroll back through the schedule to see if it has a reason? That's what I normally do. It might have been bad guide data and might show as "not new" or something.
That has happened to several of our recordings in the past couple of weeks. It didn't record an ep of AI, yet got all the rest with the same timer and a stand alone timer just vanished. There isn't any record of these 2 missed recordings. Also getting Black Screens of Death lately, started in the same time frame.
I had a strange recording issue on Monday when Chuck and 24 both had 2 hour finales. Wife also records Dancing With The Stars. That show recorded, but Chuck got split into 120 1 minute recordings and 24 didn't record. I figured 24 was because of priority, but the history said it was because of "service shutoff". It also showed both Chuck and 24 actively being recorded on tuner 2. I ended up watching both online.