Now that you mention it, there have been multiple times when the timers did not trigger, and there were about 3 (maybe more) different scenarios in which they did not work.
1) Daylight Saving Time - cured the following day
2) OTA Channels - 622 stopped recognizing the programs (later cured after a software update)
3) Maximum Timers reached - (not maximum timers, but maximum shows -either skipped or set to record-. This is particularly bad when you have a lot of timers set and the shows repeat ALL the time like MTV, Bravo, Spike, etc.
Best advice on working around this is to set a Weekly Timer, instead of ---All New Shows- Timer on the network channels that repeat shows daily.
However, as kesslerm stated, I have not seen this problem much lately either, I believe they raised the limit on Max Timers and Max Shows a while back.