For the past (almost) 2 years that I have been doing programming contests, I've noticed the trend that I have never been able to solve a single problem in the last 1/6 of the given time in any competition (e.g. last 20 minutes of a CF round or last hour of an ACM round).

For the first 1.5 years, I thought it was due to a knowledge gap, but more recently it seems that I just keep hitting some kind of invisible wall (to psychology people: I did not realize this phenomenon until yesterday, when someone pointed it out to me, so I have never went into the last hour thinking I would not solve anything).