Sometimes when people get an advantage they relax and so they start playing worse.
Sometimes they're so confident (or perhaps nervous) that they "should" win they start to play passively or with mistakes.
I'm not sure that it's any different for beginners. As long as your opponent is about the same level you'll usually win more often when material up near the end of the game. Of course time trouble is an issue too.
I'm noticing quite a pattern. In the beginning and middle game, there have been quite a few games (this is standard 15/10 time controls) where I was up some material (maybe 1 or 2 pawns) yet I cannot convert the win. It's getting to the point where even if I'm up, I don't feel as confident. I was even up a rook and lost.
I will admit that I've been under time pressure in standard 15/10 so maybe I need longer time controls.
What does everyone else think? Is this pretty common for beginners to struggle closing out games when in a winning position? BTW, I'm anywhere between 1180 and 1220 rating give or take. Thanks.