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.
very true about relaxing a little too much.
that being said, after this post I played a 1300 player and had him beat pretty good but I damn blundered a rook. so frustrating but I was able to draw the game at least.
A 1 pawn advantage can be quite hard to convert, and even 2 pawns up can end in a draw in some positions.
However, being a full rook up should be easy to convert unless there was some major positional compensation or counterplay that your opponent had that you didn't mention.
Time pressure should not be relevant either when you're a full rook up with a 10 second increment.
So it might come down to a problem with your technique, although it is hard to say without seeing the actual game.
Could you show the game(s) you were talking about to us ?
It's very normal for various reasons. If you constantly find yourself losing or drawing endgames when you should be winning them, put effort into that part of the game. For example, Silman's EndGame book is great and grows with you as it's tailored information based on your rating. There are others too, John Nunn, Bruce Pandolfini- all authors of great endgame based books. Learn enough that you can be confident in your endgame play and bring home the win!
If I have two pawns and the opponent has none, I will definitely win with style. If my opponent has a newly promoted queen, he doesn't know how to begin the checkmate, and his time on the clock easily runs out.
The endgame is quite different from the opening and middlegame. Strange, counter-intuitive ideas like the opposition, stalemate, pawn promotion, and the king as a strong, attacking piece come into play. Those take some getting used to.
Plus, as a beginner you don't reach competitive endgames very often, so you lack experience with those positions.
It's definitely worth finding an endgame book you like and dipping into it, especially for positions you encountered in your games.
I might add that strong class players and better often struggle with the endgame too. Partly because they don't study endgames and partly because endgames can be hard.
If I have two pawns and the opponent has none, I will definitely win with style.
Go for it. White to move in all positions.
to steve,
it was a complete silly blunder so not much to analyze. just a really bad brain fart. I also think that right after this post, I was pretty aggressive and I overlooked it. it sucked b/c I had time to think too!
I will admit that even though I'm like 3-5-1 draw in the past 9 standard games, I'm feeling like I'm getting a little better. I stopped playing bullet, which has also helped.
thanks everyone for your comments and suggestions. hopefully I can start converting more wins.
If you haven't seen these kinds of positions it's probably surprising that all 5 are draws 
If I have two pawns and the opponent has none, I will definitely win with style.
One word. Yes!
What you need to do, is find a good endgame book (it should cover all the basics. Opposition, basic rook ending principles, etc.) and buckle down and read it. But before you do it seems you need to know something. Endgame knowledge imo can be broken down into three categories. Specific position, general positions rules, and basic converting rules.
Specific knowledge would include the following position.
I use Zugzwang, a tactic for one pawn remaining, no minor pieces.
Dear God, that's not normal at all. You'd best quit chess forever and save yourself further embarrassment. 
The longer the time controls,the more important it becomes to be strong in the endgame.Even more so in the past,when games were adjourned.Since the endgame is an especially delicate phase, where mistakes can be of more serious consequences than in the opening for example,I would say that fast time controls-blitz in particular- really downgrade it, as most of the time the players will have one minute or so on their clock.Like a tightrope walker attacked by bees so to speak.In the past,with the longer time controls,the endgame was King.Nowadays,it's more about the opening and the middlegame in my opinion.
thanks cp6003. in your first example, I actually saw a video from chessnetwork on youtube which covered this similar concept but it's good to always reinforce good basics b/c sometimes I will just mess things up.
do you happen to have any diagrams where it's just king and pawns with maybe 1 or a 2 pawn advantage vs your opponent? these seem to be a weakness for me. if not, thanks anyways!
At our level Gumby, any advantage gained can be quickly lost by poor play. Longer time controls help, focus on consolidating helps too. So, if you gain a piece through a tactic or an opponent blunder, spend a little while figuring out the in and outs of your position as if still equal. Finally, an opponent who is objectively losing has nothing to lose and often plays better (at least I tend to, however futile)
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.